Synthesis, structure and pyrolysis of stabilised phosphonium ylides containing saturated oxygen heterocycles

Abstract

A range of twelve stabilised phosphonium ylides containing tetrahydrofuran, tetrahydropyran or 2,2-dimethyl-1,3-dioxolane rings have been prepared and fully characterised, including one X-ray structure determination of each type. The X-ray structures confirm the P=C and C=O functions to be syn and all the compounds undergo thermal extrusion of Ph₃PO to give the corresponding alkynes. In some cases there is also competing loss of Ph₃P to give different carbene-derived products and evidence has been obtained for the generation of 2-phenyloxete in this way. Raising the pyrolysis temperature leads in several cases to new secondary reactions of the alkyne products involving a sequence of alkyne to vinylidene isomerisation, intramolecular CH insertion, and retro Diels Alder reaction.

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Introduction

Thermal extrusion of Ph₃PO from suitably substituted β-oxophosphonium ylides in the so-called “intramolecular Wittig reaction” is a versatile method of alkyne synthesis.¹ The ylides are readily prepared in a few simple steps from alkyl halides and acid chlorides as stable crystalline solids, and overall this represents an unusual way to construct the alkyne triple bond (Scheme 1). The process was first described in 1959 for R¹ = R² = Ph where simply heating the ylide at 300 °C resulted in the desired elimination of Ph₃PO to give diphenylacetylene in 59% yield.² Shortly thereafter a convenient synthesis of acetylenic esters by pyrolysis of the ylides with R¹ = CO₂Me was reported.³ Over the next 25 years many further examples appeared, but using conventional pyrolysis the scope was limited to examples where R¹ was an electron withdrawing group.⁴ This restriction was overcome by the use of flash vacuum pyrolysis (FVP) which allowed synthesis of purely aliphatic and terminal alkynes from ylides with R¹ = alkyl or H.⁵ Using this method, a wide range of functionalised alkynes have been prepared and typical examples include the synthesis of fused ring heterocycles via cascade cyclisation^6,7 and chiral amino acid-derived alkynes.⁸ FVP is a technique of increasing importance in the synthesis of heterocyclic compounds,⁹ but has not so far seen any significant application in carbohydrate chemistry. In fact the use of FVP in carbohydrate chemistry is effectively limited to a single example: the pyrolytic elimination of acetic acid to form a derivative of zanamivir (Relenza).¹⁰ In this paper we describe the preparation and pyrolysis behaviour of a series of ylides containing tetrahydrofuran, tetrahydropyran and 2,2-dimethyl-1,3-dioxolane rings to check the compatibility of these ring systems common in carbohydrates with the conditions required for alkyne formation. There has been considerable interest in acetylenic carbohydrate derivatives, both in their own right,¹¹ and as intermediates in synthesis of modified nucleosides.^12,13 Alkyne-containing 2,2-dimethyl-1,3-dioxolanes have also found use in total synthesis.^14,15 There is a single previous example of this approach in which a tetrahydropyran-containing ylide 1 was prepared and subjected to conventional pyrolysis at 200–250 °C (heating the solid above its mp) to afford alkyne 2 in 75% yield (Scheme 2).¹⁶

Scheme 1

Scheme 2

Results and discussion

The ylides 4 were prepared by two different methods depending on the nature of R¹ (Scheme 3). For monostabilised ylides (R¹ = H, Me, Ph) reaction of the appropriate phosphonium salt 3 with butyllithium in THF followed by the acid chloride proceeded with “transylidation”¹⁷ requiring a 2 : 1 reacting ratio, while for distabilised ylides (R¹ = COMe, COPh, CO₂Et) reaction of 5 with the acid chloride and triethylamine in toluene was used.¹⁸ Tetrahydrofuran-2-carbonyl chloride was prepared by reaction of the commercially available acid with oxalyl chloride, tetrahydropyran-4-carbonyl chloride was prepared by hydrolysis of methyl tetrahydropyran-4-carboxylate using sodium hydroxide followed by reaction with thionyl chloride, while 2,2-dimethyl-1,3-dioxolane-4-carbonyl chloride was prepared from mannitol diacetonide by sodium periodate cleavage¹⁹ followed by potassium permanganate oxidation of the resulting aldehyde and treatment of the potassium carboxylate with oxalyl chloride.²⁰ The 12 ylides 6–17 were obtained in low to moderate yield as colourless or pale yellow crystals (Scheme 3). They all gave ³¹P NMR chemical shifts in the range δ_P +14.9–17.9 and the ¹³C NMR spectra showed a highly consistent and informative pattern of phosphorus coupling. X-Ray diffraction has previously been used to obtain valuable information on the structure of stabilised ylides, particularly in respect of the P=C–C=O torsion angle which is important for successful pyrolytic alkyne formation.²¹ Since no X-ray structures of ylides containing saturated oxygen heterocycles are known, we have determined the structure of one representative example from each of the three ring systems.

Scheme 3 Synthetic routes used and new ylides prepared.

The structure of compound 10 (Fig. 1) shows an almost planar tetrahydrofuran ring and both carbonyl groups syn to the P=C bond. However the degree of delocalisation in the sense P⁺–C=C–O⁻ as indicated by the CC and CO bond lengths is much greater for the CO–THF function with a torsion angle of 0.7° than for the COPh with a torsion angle of 34°.

Fig. 1 X-ray structure of 10 showing numbering scheme. Selected bond lengths and torsion angles: P(1)–C(7) 1.752(2), C(7)–C(8) 1.459(3), C(8)–O(8) 1.232(2), C(7)–C(6) 1.423(3), C(6)–O(6) 1.240(2) Å; P(1)–C(7)–C(8)–O(8) 34.2(2), P(1)–C(7)–C(6)–O(6) 0.7(2)°.

In the tetrahydropyran series, the structure of the monostabilised ylide 13 was determined (Fig. 2) and this shows the tetrahydropyran in an almost perfect chair conformation with the oxo ylide function equatorial. The bond lengths again show the oxo ylide function to be substantially delocalised in the sense P⁺–C=C–O⁻ and P=C and C=O are syn.

Fig. 2 X-ray structure of 13 showing numbering scheme. Selected bond lengths and torsion angle: P(1)–C(19) 1.729(2), C(19)–C(20) 1.396(2), C(20)–O(20) 1.267(2) Å; P(1)–C(19)–C(20)–O(20) 2.4(2)°.

The structure of 17 proved to be more complex with two separate and slightly different molecules in the unit cell (Fig. 3). In each case the ester carbonyl is anti to the P=C bond while the dioxolanyl ketone is syn to it but, while the molecule containing P(41) has these functions almost coplanar (torsion angles 178.7 and 1.0°), the geometry for the other molecule containing P(1) is less ideal (torsion angles 156.8 and 10.6°).

Fig. 3 X-ray structure of 17 showing numbering scheme. Selected bond lengths and torsion angles: P(1)–C(19) 1.751(5), C(19)–C(24) 1.434(6), C(24)–O(24) 1.227(7), C(19)–C(20) 1.445(9), C(20)–O(20) 1.208(7), P(41)–C(59) 1.760(6), C(59)–C(60) 1.445(9), C(60)–O(60) 1.215(7), C(59)–C(64) 1.444(6), C(64)–O(64) 1.224(7) Å; P(1)–C(19)–C(24)–O(24) 10.6(7), P(1)–C(19)–C(20)–O(20) 156.8(4), P(41)–C(59)–C(60)–O(60) 178.7(4), P(41)–C(59)–C(64)–O(64) 1.0(7)°.

The behaviour of the compounds was now studied under flash vacuum pyrolysis conditions. The apparatus used has been described previously,²² and based on our extensive experience with oxo stabilised ylides, temperatures in the range 500–750 °C were used with a pressure of 1–5 × 10⁻² Torr. The two compounds with a hydrogen atom on the ylidic bond, 6 and 12, reacted completely at 750 °C with clean extrusion of Ph₃PO being observed in each case (Scheme 4). For 6 the other product was the expected terminal alkyne 18 (36%) together with a little furan and benzene from thermal degradation. In the case of the tetrahydropyran compound 12, the expected alkyne 19 (42%) was again formed but this was now accompanied by low yields of cyclopentadiene and acetaldehyde. We believe these to result from the mechanism shown in Scheme 4 in which the alkyne isomerises to the vinylidene 20 and this undergoes intramolecular CH insertion at the 2-position of the tetrahydropyran to form the bicyclic product 21. This is then set up to undergo a retro Diels–Alder reaction with formation of cyclopentadiene and oxirane, which isomerises to acetaldehyde. The formation of vinylidenes such as 20 by rearrangement of alkynes under FVP conditions is well known,²³ and will be encountered again in the FVP of 9 and 10.

Scheme 4

The ylide 7 with a methyl group on the ylidic position reacted completely at 730 °C to give a 2 : 1 mixture of Ph₃P and Ph₃PO at the furnace exit. In the cold trap the expected alkyne 22 (20%) from loss of Ph₃PO was accompanied by 2-propionyl-4,5-dihydrofuran 24 (30%) (Scheme 5). We propose that this product is formed by loss of Ph₃P to give the (triplet) carbene 23, which then undergoes twofold intramolecular hydrogen atom abstraction to afford the product. In contrast the corresponding tetrahydropyran ylide 13 underwent exclusive elimination of Ph₃PO at 710 °C to give the expected alkyne 25 in good yield as the only product. The extrusion of a phosphine as opposed to phosphine oxide from β-oxophosphonium ylides is very unusual, but has been observed in the FVP of a benzotriazolyl ylide,^24,25 and also upon photolysis of Ph₃P=CHCOPh.²⁶

Scheme 5

The two compounds 8 and 14 with a phenyl substituent on the ylidic carbon both reacted completely at 700 °C with exclusive elimination of Ph₃PO. With the tetrahydrofuran compound 8 the product was the known alkyne 26 (56%) (Scheme 6), which showed good agreement with literature spectroscopic data,^27,28 while for the tetrahydropyran compound 14, the expected alkyne product 27 (46%) was accompanied by the isomeric allene 28 (20%). The ylide 16 containing phenyl and 2,2-dimethyl-1,3-dioxolane groups was found to undergo loss of both Ph₃PO and Ph₃P to give separate products with the relative importance of the two competing pathways varying with temperature (Scheme 7). Thus at 650 °C, there was predominant loss of Ph₃PO (Ph₃PO/Ph₃P 8 : 1) to afford the expected alkyne 29 in 48% yield readily identified by comparison with reported spectroscopic data.²⁹ By increasing the pyrolysis temperature to 750 °C the importance of the Ph₃P elimination route was increased (Ph₃PO/Ph₃P 3 : 1) and this allowed tentative identification of the resulting product as 2-phenyloxete 30. A correct HRMS measurement corresponding to 30 was obtained and, although peak overlap prevented full spectroscopic characterisation, the key signals in ¹H and ¹³C NMR spectra were consistent with a highly shielded (enol ether) =CH– [δ_H 6.16 (1 H, t, J 6.9); δ_C 93.9] adjacent to a –CH₂O– [δ_H 5.14 (2 H, d, J 6.9); δ_C 78.7] function. The latter compares well with the value of δ_H 5.44 observed for the oxete CH₂ in compound 34 (Scheme 8).³⁰

Scheme 6

Scheme 7

Scheme 8 Previously reported oxetes.

Simple oxetes such as this are almost unknown but there has been recent interest in more highly substituted examples such as the 4-methylene compounds 35 obtained by gold-catalysed cyclisation of α-hydroxybenzylallenes,³¹ and stable fully substituted compounds 36 prepared in high enantiomeric purity from ethyl trifluoropyruvate and alkynes.³² The parent compound has also been generated³³ and was stable enough for a ¹H NMR spectrum to be recorded at −25 °C [δ_H 6.70 (1 H, br s), 5.73 (1 H, br s) and 5.27 (2 H, br s)], but it isomerised to acrolein on warming to room temperature.

We rationalise the formation of 30, as shown in Scheme 7, by initial loss of Ph₃P from ylide 16 to give the carbene 31 which undergoes intramolecular oxonium ylide formation to form 32. This can then rearrange as shown to give 33, which loses CO and acetone to afford the oxete. Further support for this route was provided by the observation of acetone among the pyrolysis products.

We next examined the two ylides 9 and 10 where there is potentially a choice between elimination of Ph₃PO in two directions to give isomeric alkynyl ketones. In previous such cases there has been little selectivity with almost equal proportions of the two possible products being formed.^34,35 This also proved to be the case here, with compound 9 undergoing complete extrusion of Ph₃PO at 500 °C to afford a mixture of 37 (26%) and 39 (58%), while the benzoyl compound 10 gave a mixture of 38 (29%) and 40 (30%) under the same conditions (Scheme 9). All these alkynyl ketone products are previously unknown.

Scheme 9

When ylides 9 and 10 were pyrolysed at the higher temperature of 700 °C an interesting new process was observed. In each case the previously observed alkynyl ketones were still formed but these were accompanied by the 3-acylfurans 41 and 42, readily identified by comparison with literature spectroscopic data.^36,37 Thus FVP of 9 at 700 °C gave 37 (18%), 39 (33%) and 41 (17%) while under the same conditions 10 gave 38 (12%), 40 (14%) and 42 (37%). As shown we propose that this involves isomerisation²³ of the alkynyl ketones 39 and 40 to the vinylidene 43 which undergoes intramolecular insertion into the marked CH bond to give the oxabicyclic compounds 44 which are ideally set up to undergo retro-Diels Alder elimination of ethene to afford the observed acylfuran products.

Pyrolysis of ylides containing both adjacent ester and ketone carbonyl functions is well known to result in elimination of Ph₃PO only from the latter to give acetylenic esters. The method was first developed by Märkl using conventional pyrolysis of methyl esters,³ but was later shown to also be amenable to FVP conditions using ethyl esters.³⁸ An added advantage of using the latter technique is that, while efficient Ph₃PO elimination to give the acetylenic esters occurs at 500 °C, simply increasing the temperature to 750 °C additionally leads to loss of the whole ester group to afford the alk-1-ynes. Mechanistic studies of this unusual process have been described.³⁹ In agreement with this pattern, when the three ester-containing ylides 11, 15, and 17 were subjected to FVP, mixtures of the acetylenic esters and the alk-1-ynes were obtained with the ratio depending on the temperature (Scheme 10). Thus at 500 °C, FVP of 11 gave ester 45 (42%) together with the alkyne 18 (16%). At 750 °C, there was extensive decomposition with only products such as furan and cyclopentadiene isolated in low yield. FVP of the tetrahydropyan ylide 15 at 500 °C gave the ester 46 in 60% yield, while increasing the furnace temperature to 750 °C led to alkyne 19 (30%) together with the decomposition products cyclopentadiene (20%) and acetaldehyde (25%) formed from 19 as shown in Scheme 4.

Scheme 10

FVP of 17 at 600 °C gave mainly the ester 47 (68%) with a little of the alkyne 48 (9%) while at 650 °C the ratio of products had changed to 47 (39%) and 48 (30%). It therefore appears that lower temperature FVP of these three ylides provides an effective route to the acetylenic esters but for the alk-1-ynes FVP of ylides such as 6 and 12 is preferable.

Experimental

Instrumentation, general techniques and starting materials

Melting points were recorded on a Kofler hot-stage microscope and are uncorrected. Infra red spectra were recorded as Nujol mulls for solids and as thin films for liquids on a Perkin Elmer 1420 instrument. NMR spectra were obtained for ¹H at 300, 400 or 500 MHz, for ¹³C at 75, 100 or 125 MHz and for ³¹P at 121 MHz all using Bruker instruments. All spectra were run on solutions in CDCl₃ with internal Me₄Si as reference for ¹H and ¹³C and external H₃PO₄ for ³¹P. Chemical shifts are reported in ppm to high frequency of the reference and coupling constants J are in Hz. Mass spectra were obtained on a Micromass LCT spectrometer using electrospray ionisation.

Tetrahydrofuran-2-carbonyl chloride was prepared (87%) by reaction of the commercially available acid with oxalyl chloride, tetrahydropyran-4-carbonyl chloride was prepared by hydrolysis of methyl tetrahydropyran-4-carboxylate using sodium hydroxide (quant.) followed by reaction with thionyl chloride (84%), while 2,2-dimethyl-1,3-dioxolane-4-carbonyl chloride was prepared from mannitol diacetonide by sodium periodate cleavage¹⁹ followed by potassium permanganate oxidation of the resulting aldehyde and treatment of the potassium carboxylate with oxalyl chloride.²⁰

General procedure A for synthesis of monostabilised ylides

A suspension of the appropriate phosphonium salt (1 equiv.) in dry THF was stirred at rt under nitrogen while a solution of n-BuLi in hexanes (2.5 M, 1 equiv.) was added dropwise by syringe. After the addition the mixture was stirred for 30 min and then a solution of the acid chloride (0.5 equiv.) in dry THF was added dropwise. The mixture was stirred for 12 h and then partitioned between water and diethyl ether. The organic extract was dried (MgSO₄) and evaporated to give the product. If necessary a little EtOAc was added to induce crystallisation and the products were recrystallised from EtOAc.

General procedure B for synthesis of distabilised ylides

A solution of the appropriate stabilised ylide (1 equiv.) and triethylamine (1 equiv.) in dry toluene (15 cm³) was stirred at rt while a solution of the acid chloride (1 equiv.) in dry toluene (5 cm³) was added dropwise. After stirring for 12 h, the mixture was filtered to remove triethylamine hydrochloride and the filtrate was evaporated to give the product. If necessary a little EtOAc was added to induce crystallisation and the products were recrystallised from EtOAc.

Tetrahydrofuran-containing ylides

1-(Tetrahydrofuran-2-yl)-2-(triphenylphosphoranylidene)ethan-1-one 6. Reaction using procedure A with methyltriphenylphosphonium bromide (10.62 g, 29.7 mmol) in THF (50 cm³), 2.5 M n-BuLi (12.34 cm³, 29.7 mmol) and tetrahydrofuran-2-carbonyl chloride (2.00 g, 14.86 mmol) in THF (20 cm³) gave the product (1.97 g, 35%) as pale yellow crystals, mp 187–189 °C (from EtOAc) (Found 375.1504. C₂₄H₂₄O₂P (M + H) requires 375.1514); ν_max/cm⁻¹ 1531 (CO), 1102, 1072, 872, 758, 716 and 696; δ_H (300 MHz) 7.70–7.60 (6 H, m), 7.60–7.50 (3 H, m), 7.50–7.40 (6 H, m), 4.40–4.30 (1 H, m, 2-H), 4.16 (1 H, d, J 26.3, CH=P), 4.10–4.00 (1 H, m, 5-H), 3.90–3.80 (1 H, m, 5-H), 2.30–2.15 (1 H, m) and 2.10–1.80 (3 H, m); δ_C (100 MHz) 193.6 (d, J 3, CO), 133.0 (d, J 10, C-2 of Ph), 132.0 (d, J 1, C-4 of Ph), 128.8 (d, J 12, C-3 of Ph), 127.0 (d, J 90, C-1 of Ph), 83.0 (d, J 13, 2-CH), 68.9 (5-CH₂), 48.5 (d, J 109, P=CH), 30.9 (3-CH₂) and 25.7 (4-CH₂); δ_P (121 MHz) +16.2; m/z (ESI) 771.28 (2M + Na, 7%), 397.13 (M + Na, 12) and 375.15 (M + H, 100).

1-(Tetrahydrofuran-2-yl)-2-(triphenylphosphoranylidene)propan-1-one 7. Reaction using procedure A with ethyltriphenylphosphonium bromide (5.6 g, 15 mmol) in THF (40 cm³), 2.5 M n-BuLi (6.0 cm³, 15 mmol) and tetrahydrofuran-2-carbonyl chloride (1.0 g, 7.43 mmol) in THF (10 cm³) gave the product (1.20 g, 41%) as colourless crystals, mp 115–117 °C (from EtOAc) (Found 389.1620. C₂₅H₂₆O₂P (M + H) requires 389.1670); ν_max/cm⁻¹ 1520 (CO), 1105, 923, 750, 711 and 693; δ_H (300 MHz) 7.60–7.39 (15 H, m), 4.87 (1 H, td, J 7.2, 1.2, 2-H), 4.04–3.97 (1 H, m, 5-H), 3.90–3.83 (1 H, m, 5-H), 2.15–2.07 (1 H, m), 2.00–1.60 (3 H, m) and 1.68 (3 H, d, J 16.5, Me); δ_C (75 MHz) 188.4 (d, J 4, CO), 133.4 (d, J 10, C-2 of Ph), 131.3 (d, J 3, C-4 of Ph), 128.5 (d, J 12, C-3 of Ph), 127.3 (d, J 90, C-1 of Ph), 78.8 (d, J 14, 2-CH), 68.8 (5-CH₂), 55.7 (d, J 107, P=C), 29.3 (3-CH₂), 25.9 (4-CH₂) and 12.3 (d, J 12, Me); δ_P (121 MHz) +17.9; m/z (ESI) 389.16 (M + H, 100).

2-Phenyl-1-(tetrahydrofuran-2-yl)-2-(triphenylphosphanylidene)ethan-1-one 8. Reaction using procedure A with benzyltriphenylphosphonium chloride (5.78 g, 14.86 mmol) in THF (20 cm³), 2.5 M n-BuLi (6.0 cm³, 15.0 mmol) and tetrahydrofuran-2-carbonyl chloride (1.0 g, 7.43 mmol) in THF (10 cm³) gave the product (0.78 g, 23%) as pale yellow crystals, mp 179–182 °C (from EtOAc) (Found 451.1809. C₃₀H₂₈O₂P (M + H) requires 451.1827); ν_max/cm⁻¹ 1530 (CO), 1223, 1106, 1068, 746, 701 and 693; δ_H (300 MHz) 7.57–7.42 (9 H, m), 7.38–7.32 (6 H, m), 7.05–6.95 (5 H, m), 4.58 (1 H, t, J 6.4, 2-H), 4.00–3.92 (1 H, m, 5-H), 3.80–3.73 (1 H, m, 5-H), 1.99–1.82 (3 H, m) and 1.73–1.65 (1 H, m); δ_C (125 MHz) 189.0 (d, J 5, CO), 137.2 (d, J 11, C-1 of Ph), 135.0 (d, J 5, C-2 of Ph), 133.6 (d, J 10, C-2 of PPh), 131.4 (d, J 3, C-4 of PPh), 128.4 (d, J 12, C-3 of PPh), 127.5 (d, J 1, C-3 of Ph), 126.8 (d, J 90, C-1 of PPh), 125.2 (d, J 3, C-4 of Ph), 78.2 (d, J 12, 2-CH), 70.2 (d, J 106, P=C), 69.2 (5-CH₂), 30.4 (3-CH₂) and 26.2 (4-CH₂); δ_P (121 MHz) +15.5; m/z (ESI) 923.34 (2M + Na, 42%), 473.16 (M + Na, 6) and 451.18 (M + H, 100).

1-(Tetrahydrofuran-2-yl)-2-(triphenylphosphoranylidene)butane-1,3-dione 9. Reaction using procedure B with triphenylphosphoranylideneacetone (2.37 g, 7.43 mmol), triethylamine (1.03 cm³, 0.75 g, 7.43 mmol) and tetrahydrofuran-2-carbonyl chloride (1.00 g, 7.43 mmol) in toluene (20 cm³) gave the product (2.4 g, 77%) as colourless crystals, mp 127.5–129.5 °C (from EtOAc) (Found 439.1421. C₂₆H₂₅NaO₃P (M + Na) requires 439.1439); ν_max/cm⁻¹ 1553 (CO), 1108, 1066, 757 and 698; δ_H (300 MHz) 7.70–7.60 (6 H, m), 7.50–7.35 (9 H, m), 4.86 (1 H, dd, J 7.5, 5.4, 2-H), 3.89–3.82 (1 H, m, 5-H), 3.76–3.69 (1 H, m, 5-H), 2.15–2.00 (2 H, m), 2.09 (3 H, d, J 0.6, Me) and 1.82–1.72 (2 H, m); δ_C (75 MHz) 196.2 (d, J 7, 1-CO), 192.7 (d, J 10, 3-CO), 133.0 (d, J 10, C-2 of Ph), 131.6 (d, J 3, C-4 of Ph), 128.5 (d, J 12, C-3 of Ph), 126.3 (d, J 91, C-1 of Ph), 86.0 (d, J 100, P=C), 80.9 (d, J 6, 2-CH), 68.8 (5-CH₂), 30.1 (d, J 4, Me), 29.6 (3-CH₂) and 25.3 (4-CH₂); δ_P (121 MHz) +15.6; m/z (ESI) 855.30 (2M + Na, 12%), 439.14 (M + Na, 100) and 417.16 (M + H, 18).

1-Phenyl-3-(tetrahydrofuran-2-yl)-2-(triphenylphosphoranylidene)propane-1,3-dione 10. Reaction using procedure B with triphenylphosphoranylideneacetophenone (2.83 g, 7.43 mmol), triethylamine (1.03 cm³, 0.75 g, 7.43 mmol) and tetrahydrofuran-2-carbonyl chloride (1.00 g, 7.43 mmol) in toluene (20 cm³) gave the product (0.84 g, 24%) as pale yellow crystals, mp 179–181 °C (from EtOAc) (Found 479.1762. C₃₁H₂₈O₃P (M + H) requires 479.1776); ν_max/cm⁻¹ 1546 (CO), 1317, 1105, 1061, 713 and 694; δ_H (300 MHz) 7.70–7.60 (8 H, m), 7.50–7.25 (12 H, m), 4.54 (1 H, t, J 6.7, 2-H), 3.85–3.70 (1 H, m, 5-H), 3.65–3.55 (1 H, m, 5-H), 1.85–1.75 (2H, m) and 1.70–1.57 (2 H, m); δ_C (75 MHz) 195.0 (d, J 3, CO), 192.8 (d, J 10, CO), 143.4 (d, J 8, C-1 of COPh), 133.2 (d, J 10, C-2 of PPh), 131.7 (d, J 3, C-4 of PPh), 130.8 (C-4 of COPh), 128.8 (2CH of COPh), 128.5 (d, J 12, C-3 of PPh), 128.1 (2CH of COPh), 125.7 (d, J 92, C-1 of PPh), 83.3 (d, J 100, P=C), 79.7 (d, J 9, 2-CH), 69.1 (5-CH₂), 30.1 (3-CH₂) and 25.4 (4-CH₂); δ_P (121 MHz) +17.5; m/z (ESI) 979.33 (2M + Na, 62%), 501.16 (M + Na, 55) and 479.18 (M + H, 100).

Ethyl 3-oxo-3-(tetrahydrofuran-2-yl)-2-(triphenylphosphoranylidene)propanoate 11. Reaction using procedure B with ethyl triphenylphosphoranylideneacetate (2.58 g, 7.43 mmol), triethylamine (1.03 cm³, 0.75 g, 7.43 mmol) and tetrahydrofuran-2-carbonyl chloride (1.00 g, 7.43 mmol) in toluene (20 cm³) gave the product (0.82 g, 25%) as colourless crystals, mp 175–177 °C (from EtOAc) (Found 447.1712. C₂₇H₂₈O₄P (M + H) requires 447.1725); ν_max/cm⁻¹ 1654 (CO), 1575 (CO), 1299, 1105, 1082, 750 and 692; δ_H (300 MHz) 7.70–7.60 (6 H, m), 7.58–7.38 (9 H, m), 5.51 (1 H, ddd, J 8.2, 5.2, 1.0, 2-H), 3.95–3.80 (2 H, m, 5-H), 3.80–3.65 (2 H, m, CH₂ of Et), 2.50–2.38 (1 H, m), 2.10–1.90 (1H, m), 1.90–1.75 (2 H, m) and 0.67 (3 H, t, J 7.1, CH₃ of Et); δ_C (100 MHz) 196.6 (d, J 3, 3-CO), 167.4 (d, J 15, 1-CO), 133.0 (d, J 10, C-2 of Ph), 131.5 (d, J 2, C-4 of Ph), 128.4 (d, J 12, C-3 of Ph), 126.5 (d, J 93, C-1 of Ph), 80.7 (d, J 9, 2-CH), 69.1 (5-CH₂), 68.8 (d, J 109, P=C), 58.4 (OEt), 30.9 (3-CH₂), 25.2 (4-CH₂) and 13.7 (OEt); δ_P (121 MHz) +17.1; m/z (ESI) 915.32 (2M + Na, 20%) and 447.17 (M + H, 100).

Tetrahydropyran-containing ylides

1-(Tetrahydropyran-4-yl)-2-(triphenylphosphoranylidene)ethanone 12. Reaction using procedure A with methyltriphenylphosphonium bromide (9.61 g, 26.9 mmol) in THF (50 cm³), 2.5 M n-BuLi (11.17 cm³, 26.9 mmol) and tetrahydropyran-4-carbonyl chloride (2.00 g, 13.45 mmol) in THF (20 cm³) gave the product (1.21 g, 23%) as pale orange crystals, mp 162–164 °C (from EtOAc) (Found 389.1659. C₂₅H₂₆O₂P (M + H) requires 389.1670); ν_max/cm⁻¹ 1540 (CO), 1400, 1110, 1082, 860, 752, and 697; δ_H (300 MHz) 7.70–7.60 (6 H, m), 7.60–7.50 (3 H, m), 7.50–7.40 (6 H, m), 4.10–3.95 (2 H, m), 3.70 (1 H, d, J 25.8, CH=P), 3.50–3.35 (2 H, m), 2.45–2.30 (1 H, m, 4-H) and 1.90–1.75 (4 H, m); δ_C (100 MHz) 195.1 (d, J 1, CO), 132.9 (d, J 10, C-2 of Ph), 131.9 (d, J 1, C-4 of Ph), 128.8 (d, J 12, C-3 of Ph), 127.2 (d, J 90, C-1 of Ph), 68.1 (2,6-CH₂), 49.2 (d, J 107, P=CH), 46.2 (d, J 14, 4-CH) and 30.8 (3,5-CH₂); δ_P (121 MHz) +15.9; m/z (ESI) 389.17 (M + H, 100).

1-(Tetrahydropyran-4-yl)-2-(triphenylphosphoranylidene) propan-1-one 13. Reaction using procedure A with ethyltriphenylphosphonium bromide (5.00 g, 13.4 mmol) in THF (40 cm³), 2.5 M n-BuLi (5.40 cm³, 13.4 mmol) and tetrahydropyran-4-carbonyl chloride (1.00 g, 6.73 mmol) in THF (10 cm³) gave the product (0.71 g, 26%) as yellow crystals, mp 183–185 °C (from EtOAc) (Found 403.1810. C₂₆H₂₈O₂P (M + H) requires 403.1827); ν_max/cm⁻¹ 1520 (CO), 1099, 856, 754, 716 and 697; δ_H (300 MHz) 7.60–7.35 (15 H, m), 4.10–3.95 (2 H, m), 3.50 (2 H, td, J 12, 3), 2.93 (1 H, tt, J 7.8, 3.9, 4-H), 1.95–1.80 (2 H, m), 1.66 (3 H, d, J 16.2, Me) and 1.66–1.55 (2 H, m); δ_C (75 MHz) 191.2 (d, J 4, CO), 133.3 (d, J 10, C-2 of Ph), 131.2 (d, J 3, C-4 of Ph), 128.4 (d, J 12, C-3 of Ph), 128.0 (d, J 90, C-1 of Ph), 68.1 (2,6-CH₂), 55.4 (d, J 106, P=C), 40.7 (d, J 10, 4-CH), 29.2 (3,5-CH₂) and 12.7 (d, J 14, Me); δ_P (121 MHz) +16.8; m/z (ESI) 425.16 (M + Na, 12%) and 403.18 (M + H, 100).

2-Phenyl-1-(tetrahydropyran-4-yl)-2-(triphenylphosphoranylidene)ethan-1-one 14. Reaction using procedure A with benzyltriphenylphosphonium chloride (5.23 g, 13.4 mmol) in THF (20 cm³), 2.5 M n-BuLi (5.4 cm³, 13.46 mmol) and tetrahydropyran-4-carbonyl chloride (1.00 g, 6.73 mmol) in THF (10 cm³) gave the product (0.20 g, 6%) as pale yellow crystals, mp 178–182 °C (from EtOAc) (Found 465.1962. C₃₁H₃₀O₂P (M + H) requires 465.1983); ν_max/cm⁻¹ 1520 (CO), 1124, 1105, 861, 756, 709 and 691; δ_H (300 MHz) 7.60–7.40 (9 H, m), 7.40–7.30 (6 H, m), 7.05–6.95 (5 H, m), 3.95–3.85 (2 H, m), 3.24 (2 H, td, J 12, 2), 2.74 (1 H, tt, J 11.4, 3.9, 4-H), 1.95–1.75 (2 H, m) and 1.55–1.48 (2 H, m); δ_C (125 MHz) 191.4 (d, J 5, CO), 137.9 (d, J 13, C-1 of Ph), 134.8 (d, J 5, C-2 of Ph), 133.5 (d, J 10, C-2 of PPh), 131.2 (d, J 3, C-4 of PPh), 128.3 (d, J 12, C-3 of PPh), 127.6 (d, J 2, C-3 of Ph), 127.4 (d, J 90, C-1 of PPh), 125.1 (d, J 2, C-4 of Ph), 70.1 (d, J 108, P=C), 67.8 (2,6-CH₂), 41.1 (d, J 9, 4-CH) and 29.6 (3,5-CH₂); δ_P (121 MHz) +14.9; m/z (ESI) 951.37 (2M + Na, 18%), 487.18 (M + Na, 27%) and 465.20 (M + H, 100).

Ethyl 3-oxo-3-(tetrahydropyran-4-yl)-2-(triphenylphosphoranylidene)propanoate 15. Reaction using procedure B with ethyl triphenylphosphoranylideneacetate (2.34 g, 6.73 mmol), triethylamine (0.94 cm³, 0.68 g, 6.73 mmol) and tetrahydropyran-4-carbonyl chloride (0.89 g, 6.73 mmol) in toluene (20 cm³) gave the product (2.05 g, 66%) as pale yellow crystals, mp 170–171 °C (from EtOAc) (Found 461.1867. C₂₈H₃₀O₄P (M + H) requires 461.1882); ν_max/cm⁻¹ 1657 (CO), 1556 (CO), 1291, 1107, 1080, 1063, 746 and 693; δ_H (300 MHz) 7.65–7.55 (6 H, m), 7.55–7.35 (9 H, m), 4.01–3.93 (2 H, m), 3.81 (1 H, tt, J 11.4, 3.9, 4-H), 3.70 (2 H, q, J 7.1, OEt), 3.54 (2 H, td, J 11.3, 3.0), 1.85–1.60 (4 H, m, 3,5-H) and 0.63 (3 H, t, J 7.1, OEt); δ_C (100 MHz) 198.9 (d, J 5, 3-CO), 167.5 (d, J 15, 1-CO), 132.9 (d, J 10, C-2 of Ph), 131.5 (d, J 3, C-4 of Ph), 128.4 (d, J 12, C-3 of Ph), 126.9 (d, J 93, C-1 of Ph), 69.8 (d, J 109, P=C), 68.0 (2,6-CH₂), 58.3 (OEt), 43.3 (d, J 7, 4-CH), 29.4 (3,5-CH₂) and 13.7 (OEt); δ_P (121 MHz) +17.5; m/z (ESI) 943.35 (2M + Na, 32%), 483.17 (M + Na, 7%) and 461.19 (M + H, 100).

1,3-Dioxolane-containing ylides

1-(2,2-Dimethyl-1,3-dioxolan4-yl)-2-phenyl-2-(triphenylphosphoranylidene)ethan-1-one 16. Reaction using procedure A with benzyltriphenylphosphonium chloride (3.31 g, 8.5 mmol) in THF (40 cm³), 2.5 M n-BuLi (3.4 cm³, 8.5 mmol) and 2,2-dimethyl-1,3-dioxolane-4-carbonyl chloride (0.70 g, 4.25 mmol) in THF (10 cm³) gave the product (0.51 g, 25%) as colourless crystals, mp 169–171 °C (from EtOAc) (Found 481.1914. C₃₁H₃₀O₃P (M + H) requires 481.1933); ν_max/cm⁻¹ 1717, 1549 (CO), 1523, 1103, 870, 761 and 694; δ_H (300 MHz) 7.60–7.40 (9 H, m), 7.40–7.30 (6 H, m), 7.00–6.95 (5 H, m), 4.75 (1 H, ddd, J 8.4, 6.9, 1.2), 3.895 (1 H, half AB pattern of d, J 7.5, 6.9), 3.86 (1 H, half AB pattern of d, J 8.4, 7.5), 1.41 (3 H, s, Me) and 1.31 (3 H, s, Me); δ_C (75 MHz) 184.3 (d, J 6, CO), 136.5 (d, J 11, C-1 of Ph), 135.0 (d, J 5, C-2 of Ph), 133.6 (d, J 10, C-2 of PPh), 131.5 (d, J 3, C-4 of PPh), 128.4 (d, J 12, C-3 of PPh), 127.7 (d, J 2, C-3 of Ph), 126.3 (d, J 91, C-1 of PPh), 125.6 (d, J 3, C-4 of Ph), 109.7 (dioxolane C-2), 76.4 (d, J 13, dioxolane C-4), 71.0 (d, J 107, P=C), 68.0 (dioxolane C-5), 26.2 (Me) and 26.0 (Me); δ_P (121 MHz) +16.1; m/z (ESI) 481.19 (M + H, 100).

Ethyl 3-(2,2-dimethyl-1,3-dioxolan-4-yl)-3-oxo-2-(triphenylphosphoranylidene)propanoate 17. Reaction using procedure B with ethyl triphenylphosphoranylideneacetate (1.47 g, 4.25 mmol), triethylamine (0.60 cm³, 0.43 g, 4.25 mmol) and 2,2-dimethyl-1,3-dioxolan-4-carbonyl chloride (0.70 g, 4.25 mmol) in toluene (10 cm³) gave the product (0.52 g, 26%) as colourless crystals, mp 154–156 °C (Found 477.1819. C₂₈H₃₀O₅P (M + H) requires 477.1831); ν_max/cm⁻¹ 1655, 1597 (CO), 1300, 1288, 1104, 1088, 842, 756 and 689; δ_H (500 MHz) 7.70–7.60 (6 H, m), 7.60–7.50 (3 H, m), 7.50–7.40 (6 H, m), 5.51 (1 H, ddd, J 7.5, 5.4, 1.2), 4.57 (1 H, dd, J 8.2, 7.5), 4.16 (1 H, dd, J 8.2, 5.4), 3.74 and 3.66 (2 H, AB pattern of q, J 10.5, 7.2), 1.39 (3 H, s, Me), 1.36 (3 H, s, Me) and 0.64 (3 H, t, J 7.2, OEt); δ_C (125 MHz) 193.2 (d, J 3, 3-CO), 167.5 (d, J 14, 1-CO), 133.1 (d, J 10, C-2 of Ph), 131.6 (d, J 3, C-4 of Ph), 128.5 (d, J 12, C-3 of Ph), 126.1 (d, J 94, C-1 of Ph), 109.9 (dioxolane C-2), 79.1 (d, J 9, dioxolane C-4), 68.94 (d, J 110, P=C), 68.89 (dioxolane C-5), 58.4 (OEt), 26.1 (Me), 25.7 (Me) and 13.6 (OEt); δ_P (121 MHz) +16.9; m/z (ESI) 975.34 (2M + Na, 35%), 499.16 (M + Na, 60), 477.18 (M + H, 100).

X-ray structure determination

Compound 10, C₃₁H₂₇O₃P, M 478.53, yellow prism. Monoclinic, space group P2₁/c, a 9.9232(13), b 21.836(2), c 11.4086(13) Å, β 96.272(4)°, V 2457.3(5) ų, Z 4, D_c 1.293 Mg m⁻³, T 173 K, 26 295 reflections, 4506 unique (R_int 0.048). R₁ 0.0484, wR₂ 0.1407, R indices based on 3994 data with I > 2σ(I), 316 parameters. Data were recorded using a Rigaku XtaLB P200, Mo Kα radiation (confocal optic, λ 0.71073 Å) and Saturn detector. The structure was solved by direct methods and refined using full-matrix least-squares methods.

Compound 13, C₂₆H₂₇O₂P, M 402.47, colourless prism. Monoclinic, space group P2₁/c, a 11.829(3), b 11.011(3), c 16.248(4) Å, β 93.341(7)°, V 2112.7(9) ų, Z 4, D_c 1.265 Mg m⁻³, T 93 K, 23 466 reflections, 3850 unique (R_int 0.0592). R₁ 0.0369, wR₂ 0.1090, R indices based on 3336 data with I > 2σ(I), 263 parameters. Data were recorded using a Rigaku XtaLB P200, Mo Kα radiation (confocal optic, λ 0.71073 Å) and Saturn detector. The structure was solved by direct methods and refined using full-matrix least-squares methods.

Compound 17, C₂₈H₂₉O₅P, M 476.51, colourless prism. Triclinic, space group P1, a 9.345(4), b 10.512(3), c 14.341(4) Å, α 98.497(2), β 106.816(11), γ 110.513(8)°, V 1213.3(7) ų, Z 2, D_c 1.304 Mg m⁻³, T 125 K, 13 591 reflections, 6259 unique (R_int 0.0836). R₁ 0.0570, wR₂ 0.1352, R indices based on 5802 data with I > 2σ(I), 619 parameters. Data were recorded using a Rigaku XtaLB P200, Cu Kα radiation (confocal optic, λ 1.54187 Å) and Saturn detector. The structure was solved by direct methods and refined using full-matrix least-squares methods.

Crystallographic data (excluding structure factors) for the structures included in this paper have been deposited at the Cambridge Crystallographic Data Centre as supplementary publication No. CCDC 1435297 (10), 1435298 (13) and 1435299 (17).

Flash vacuum pyrolysis (FVP)

This was carried out using the apparatus previously described.²² The contact time in the hot zone was estimated to be ∼10 ms. In each case a solid collected at the furnace exit which was found to be either Ph₃PO or a mixture of Ph₃PO and Ph₃P. In the cold trap the other product(s) collected as a liquid or oil and this was dissolved out using either CDCl₃ (small scale) for direct NMR analysis, or CH₂Cl₂ (large scale) for purification and characterisation by the usual methods.

FVP of ylide 6 (150 mg) at 750 °C gave at the furnace exit Ph₃PO; δ_P +29.2, and in the cold trap:

2-Ethynyltetrahydrofuran 18 (14 mg, 36%) as a colourless liquid (Found 95.0493. C₆H₇O (M − H) requires 95.0497); δ_H (300 MHz) 4.62–4.58 (1 H, m, 2-H), 4.00–3.92 (1 H, m, 5-H), 3.86–3.75 (1 H, m, 5-H), 2.43 (1 H, d, J 2.0, ≡CH) and 2.24–1.85 (m, 4H, 3,4-H); δ_C (75 MHz) 83.8 (C, –C≡), 72.5 (CH, –C≡), 67.9 (CH₂, C-5), 67.8 (CH, C-2), 33.2 (CH₂) and 25.3 (CH₂).

FVP of ylide 12 (110 mg) at 750 °C gave at the furnace exit Ph₃PO; δ_P +29.2, and in the cold trap:

A mixture containing 4-ethynyltetrahydropyran19 (13 mg, 42%) as a colourless liquid (Found: 111.0812. C₇H₁₁O (M + H) requires 111.0810); ν_max/cm⁻¹ 3293 (≡C–H), 2240 (C≡C), 1262, 1090, 1058 and 813; δ_H (400 MHz) 3.905 (2 H, ddd, J 11.6, 5.6, 3.6, 2,6-H), 3.505 (2 H, ddd, J 11.6, 8.4, 3.2, 2,6-H), 2.68–2.61 (1 H, m, 4-H), 2.11 (1 H, d, J 2.4, ≡CH), 1.88–1.80 (2 H, m, 3,5-H) and 1.74–1.64 (2 H, m, 3,5-H); δ_C (100 MHz) 86.6 (C, –C≡), 69.1 (CH, –C≡), 66.2 (CH₂), 32.0 (CH₂) and 25.9 (CH), together with cyclopentadiene (4%); δ_H 6.57 (2 H, m), 6.47 (2 H, m) and 2.99 (2 H, m); δ_C 133.1 (CH), 132.2 (CH) and 41.6 (CH₂) and acetaldehyde (3%); δ_H 9.79 (1 H, q, J 3) and 2.21 (3 H, d, J 3).

FVP of ylide 7 (200 mg) at 730 °C gave at the furnace exit a 2 : 1 mixture of Ph₃P; δ_P −5.5 and Ph₃PO; δ_P +29.2, and in the cold trap:

A colourless liquid consisting of a mixture of 2-(prop-1-ynyl)tetrahydrofuran22 (20%); δ_H (400 MHz) 4.58–4.50 (1 H, m, 2-H), 3.98–3.85 (2 H, m, 5-H), 1.90–1.80 (2 H, m), 1.75–1.70 (2 H, m) and 1.84 (3 H, d, J 1.8); δ_C (100 MHz) 79.0 (C, –C≡), 70.7 (C, –C≡), 68.3 (CH), 67.8 (CH₂), 33.3 (CH₂), 25.3 (CH₂) and 3.5 (Me) [good agreement of δ_H with lit.⁴⁰] and 2-(propionyl)-4,5-dihydrofuran24 (30%) (Found 127.0754. C₇H₁₁O₂ (M + H) requires 127.0759); δ_H (300 MHz) 5.94 (1 H, t, J 3.0), 4.47 (2 H, t, J 9.8), 2.82 (2 H, td, 9.8, 3.0), 2.65 (2 H, q, J 7.2) and 1.12 (3 H, t, J 7.2); δ_C (100 MHz) 193.0 (CO), 156.0 (=C–), 110.7 (=CH–), 70.1 (OCH₂), 32.0 (COCH₂), 30.5 (4-CH₂) and 8.0 (CH₃). Preparative TLC allowed separation of the latter in pure form.

FVP of ylide 13 (70 mg) at 710 °C gave at the furnace exit Ph₃PO; δ_P +29.2, and in the cold trap:

4-(Prop-1-ynyl)tetrahydropyran 25 (65%) (Found: 125.0969. C₈H₁₃O (M + H) requires 125.0966); ν_max/cm⁻¹ 2233 (C≡C); δ_H (300 MHz) 3.89 (2 H, ddd, J 11.7, 5.1, 3.9, 2,6-H), 3.47 (2 H, ddd, J 11.7, 8.7, 2.8, 2,6-H), 2.60–2.50 (1 H, m, 4-H), 1.81 (3 H, d, J 2.0) 1.82–1.74 (2 H, m, 3,5-H) and 1.68–1.55 (2 H, m, 3,5-H); δ_C (75 MHz) 81.7 (C, –C≡), 76.4 (C, –C≡), 66.5 (CH₂), 32.6 (CH₂), 26.3 (CH) and 3.5 (Me).

FVP of ylide 8 (80 mg) at 700 °C gave at the furnace exit Ph₃PO; δ_P +29.2, and in the cold trap:

2-(Phenylethynyl)tetrahydrofuran 26 (56%) as a colourless liquid (Found 195.0776. C₁₂H₁₂ONa (M + Na) requires 195.0786); ν_max/cm⁻¹ 2231 (C≡C), 1727, 1599, 1490, 1054, 757 and 692; δ_H (300 MHz) 7.6–7.2 (5 H, m), 4.81 (1 H, dd, J 7.2, 5.0, 2-H), 4.05–3.98 (1 H, m, 5-H), 3.89–3.82 (1 H, m, 5-H), 2.25–2.15 (1 H, m), 2.15–2.00 (2 H, m) and 2.00–1.85 (1 H, m); δ_C (75 MHz) 131.7 (2 CH), 128.22 (CH), 128.17 (2 CH), 122.8 (C), 89.0 (C, –C≡), 84.4 (C, –C≡), 68.6 (CH₂), 67.9 (CH), 33.4 (CH₂) and 25.5 (CH₂) [good agreement of δ_H ²⁷ and δ_C ²⁸ with lit.].

FVP of ylide 14 (70 mg) at 700 °C gave at the furnace exit Ph₃PO; δ_P +29.2, and in the cold trap:

A mixture of 4-(phenylethynyl)tetrahydropyran27 (46%) (Found 186.1042. C₁₃H₁₄O (M) requires 186.1045); δ_H (300 MHz) 7.45–7.25 (5 H, m), 4.00–3.89 (2 H, m, 2,6-H), 3.60–3.50 (2 H, m, 2,6-H), 2.90–2.80 (1 H, m, 4-H), 1.95–1.85 (2 H, m, 3,5-H) and 1.82–1.70 (2 H, m, 3,5-H); δ_C (75 MHz) 131.6 (2 CH), 128.2 (2 CH), 126.6 (CH), 123.6 (C), 92.2 (C, –C≡), 81.5 (C, –C≡), 66.4 (CH₂), 32.3 (CH₂) and 26.8 (CH), and 4-(phenylvinylidene)tetrahydropyran28 (20%); δ_H (300 MHz) 7.45–7.25 (5 H, m), 6.10 (1 H, quintet, J 2.1), 3.89–3.75 (2 H, m, 2,6-H), 2.38–2.33 (2 H, m, 3,5-H); δ_C (75 MHz) 200.7 (=C=), 135.2 (C), 128.6 (2 CH), 127.7 (2 CH), 126.7 (CH), 101.9 (=C<), 93.7 (=CH), 68.8 (CH₂) and 31.3 (CH₂).

FVP of ylide 16 (65 mg) at 650 °C gave at the furnace exit an 8 : 1 mixture of Ph₃PO; δ_P +29.2 and Ph₃P; δ_P −5.5, and in the cold trap:

4-Phenylethynyl-2,2-dimethyl-1,3-dioxolane 29 (48%) (Found 225.0883. C₁₃H₁₄O₂Na (M + Na) requires 225.0891); ν_max/cm⁻¹ 2234, 2212 (C≡C), 1709, 1680, 1599, 1491, 1065, 758 and 692; δ_H (300 MHz) 7.48–7.40 (2 H, m), 7.35–7.25 (3 H, m), 4.95 (1 H, t, J 6.3, 4-H), 4.24 (1 H, dd, J 8.0, 6.3, 5-H), 4.01 (1 H, dd, J 8.0, 6.3, 5-H), 1.54 (3 H, q, J 0.6) and 1.43 (3 H, q, J 0.6); δ_C (75 MHz) 131.7 (2 C), 128.6 (C-4 of Ph), 128.2 (2 C), 122.2 (C-1 of Ph), 110.3 (dioxolane C-2), 86.2 (–C≡), 85.8 (–C≡), 70.0 (dioxolane C-5), 66.0 (dioxolane C-4), 26.2 and 26.0 (CMe₂) [good agreement of δ_H and δ_C with lit.²⁷]. Acetone (20%); δ_H 2.17; δ_C 207.0 and 30.9, was also present.

FVP of ylide 16 (107 mg) at 750 °C gave at the furnace exit a 3 : 1 mixture of Ph₃PO; δ_P +29.2 and Ph₃P; δ_P −5.5, and in the cold trap a mixture of 29 (data as above), acetone, and:

2-Phenyloxete 30 (Found 133.0649. C₉H₉O (M + H) requires 133.0653); δ_H (300 MHz) 6.16 (1 H, t, J 6.9) and 5.14 (2 H, d, J 6.9); δ_C (75 MHz) 93.9 (CH) and 78.7 (CH₂). Due to peak overlap the phenyl and quaternary =C– signals could not be observed with certainty.

FVP of ylide 9 (70 mg) at 500 °C gave at the furnace exit Ph₃PO; δ_P +29.2 and in the cold trap a mixture of:

1-(Tetrahydrofuran-2-yl)but-2-yn-1-one 37 (26%); δ_H (400 MHz) 4.44 (1 H, dd, J 8.6, 5.8), 3.95–3.80 (2 H, m), 2.30–2.00 (4 H, m) and 2.07 (3 H, s); δ_C (75 MHz) 188.8 (CO), 93.7 (C, –C≡), 83.7 (CH), 78.3 (C, –C≡), 69.6 (CH₂), 29.3 (CH₂), 25.25 (CH₂) and 4.3 (CH₃) and 4-(tetrahydrofuran-2-yl)but-3-yn-2-one39 (58%); δ_H (400 MHz) 4.74 (1 H, dd, J 8.0, 4.8), 4.1–3.9 (2 H, m), 2.2–1.8 (4 H, m) and 2.35 (3 H, s); δ_C (75 MHz) 184.3 (CO), 91.0 (C, –C≡), 83.5 (C, –C≡), 68.3 (CH₂), 67.6 (CH), 32.7 (CH₂), 32.6 (CH₃) and 25.34 (CH₂).

FVP of ylide 9 (110 mg) at 700 °C gave at the furnace exit Ph₃PO; δ_P +29.2 and in the cold trap a mixture of 37 (18%), 39 (33%) data as above, and:

3-Acetylfuran 41 (17%); δ_H (300 MHz) 8.03 (1 H, dd, J 1.5, 0.9), 7.45 (1 H, dd, J 2.0, 1.5), 6.77 (1 H, dd, J 2.0, 0.9) and 2.45 (3 H, s); δ_C (75 MHz) 192.6 (CO), 147.5 (CH), 144.2 (CH), 128.0 (C), 108.5 (CH) and 27.8 (CH₃) [good agreement of δ_H and δ_C with lit.³⁶].

FVP of ylide 10 (70 mg) at 500 °C gave at the furnace exit Ph₃PO; δ_P +29.2 and in the cold trap:

A mixture of 3-phenyl-1-(tetrahydrofuran-2-yl)prop-2-yn-1-one38 (29%); δ_H (300 MHz) 7.65–7.35 (5 H, m), 4.58 (1 H, dd, J 8.7, 5.7), 4.14–3.85 (2 H, m) and 2.40–1.90 (4 H, m); δ_C (75 MHz) 83.9 (CH), 69.8 (CH₂), 29.7 (CH₂) and 25.4 (CH₂) [due to low intensity and peak overlap, the signals for CO, –C≡ and Ph could not be assigned with certainty] and 1-phenyl-3-(tetrahydrofuran-2-yl)prop-2-yn-1-one40 (30%); δ_H (300 MHz) 7.65–7.35 (5 H, m), 4.89 (1 H, dd, J 7.8, 4.8), 4.14–3.85 (2 H, m) and 2.40–1.90 (4 H, m); δ_C (75 MHz) 68.4 (CH₂), 67.9 (CH), 32.9 (CH₂) and 25.4 (CH₂) [due to low intensity and peak overlap, the signals for CO, –C≡ and Ph could not be assigned with certainty]. Aromatic CH signals for 38 and 40 were at 134.2, 133.2, 130.9, 129.6, 128.63 and 128 58.

FVP of ylide 10 (50 mg) at 700 °C gave at the furnace exit Ph₃PO; δ_P +29.2 and in the cold trap a mixture of 38 (12%), 40 (14%) data as above, and:

3-Benzoylfuran 42 (37%); δ_H (300 MHz) 7.93 (1 H, dd, J 1.5, 0.9), 7.87–7.82 (2 H, m), 7.62–7.56 (1 H, m), 7.51 (1 H, dd, J 1.8, 1.5), 7.50–7.46 (2 H, m) and 6.92 (1 H, dd, J 1.8, 0.9) [good agreement of δ_H with lit.³⁷].

FVP of ylide 11 (120 mg) at 500 °C gave at the furnace exit Ph₃PO; δ_P +29.2 and in the cold trap a mixture of:

Ethyl 3-(tetrahydrofuran-2-yl)propynoate 45 (42%) (Found 167.0703. C₉H₁₁O₃ (M − H) requires 167.0708); ν_max/cm⁻¹ 2239 (C≡C), 1717, 1368, 1254, 1052, 1031, 860 and 752; δ_H (400 MHz) 4.73 (1 H, dd, J 8.0, 4.0, 2-H), 4.23 (2 H, q, J 7.0), 3.99–3.92 (1 H, m, 5-H), 3.89–3.80 (1 H, m, 5-H), 2.30–2.20 (1 H, m, 4-H), 2.20–2.00 (2 H, m), 2.00–1.85 (1 H, m) and 1.31 (3 H, t, J 7.0); δ_C (100 MHz) 153.3 (CO), 86.9 (C, –C≡), 76.1 (C, –C≡), 68.3 (CH₂), 67.5 (CH), 62.0 (CH₂), 32.7 (CH₂), 25.3 (CH₂) and 13.9 (CH₃) and 2-ethynyltetrahydrofuran18 (16%), data as for FVP of 6.

FVP of ylide 11 (120 mg) at 750 °C gave at the furnace exit Ph₃PO: δ_P +29.2 and in the cold trap a mixture containing low yields of furan, cyclopentadiene and acetaldehyde.

FVP of ylide 15 (100 mg) at 500 °C gave at the furnace exit Ph₃PO; δ_P +29.2 and in the cold trap:

Ethyl 3-(tetrahydropyran-4-yl)propynoate 46 (60%) (Found 205.0830. C₁₀H₁₄O₃Na (M + Na) requires 205.0841); ν_max/cm⁻¹ 2239 (C≡C), 1712, 1260, 1250, 1095, 1021 and 747; δ_H (500 MHz) 4.23 (2 H, q, J 7.0), 3.90 (2 H, ddd, J 11.6, 5.6, 3.6, 2,6-H), 3.51 (2 H, ddd, J 11.8, 8.8, 3.0, 2,6-H), 2.81–2.75 (1 H, m, 4-H), 1.91–1.84 (2 H, m, 3,5-H), 1.79–1.71 (2 H, m, 3,5-H) and 1.32 (3 H, t, J 7.0); δ_C (125 MHz) 153.8 (CO), 90.3 (C, –C≡), 73.8 (C, –C≡), 66.1 (CH₂), 62.0 (CH₂), 31.0 (CH₂), 26.1 (CH) and 14.0 (CH₃).

FVP of ylide 15 (100 mg) at 750 °C gave at the furnace exit Ph₃PO; δ_P +29.2 and in the cold trap 4-ethynyltetrahydropyran19 (30%) together with cyclopentadiene (20%) and acetaldehyde (25%); data as for FVP of 12.

FVP of ylide 17 (130 mg) at 600 °C gave at the furnace exit Ph₃PO; δ_P +29.2 and in the cold trap:

A mixture of ethyl 3-(2,2-dimethyl-1,3-dioxolan-4-yl)propynoate47 (68%) (Found 221.0782. C₁₀H₁₄O₄Na (M + Na) requires 221.0790); ν_max/cm⁻¹ 2244 (C≡C), 1716 (C=O), 1374, 1248, 1067 and 846; δ_H (300 MHz) 4.82 (1 H, dd, J 6.6, 5.6, 4-H), 4.24 (2 H, q, J 7.2), 4.21 (1 H, dd, J 8.4, 6.6, 5-H), 4.05 (1 H, dd, J 8.4, 5.6, 5-H), 1.50 (3 H, q, J 0.6), 1.39 (3 H, q, J 0.6) and 1.31 (3 H, t, J 7.2); δ_C (75 MHz) 153.0 (CO), 111.2 (dioxolane C-2), 84.2 (–C≡), 77.0 (–C≡), 69.2 (dioxolane C-5), 64.9 (dioxolane C-4), 62.2 (CH₂CH₃), 26.0 and 25.7 (CMe₂) and 13.9 (CH₂CH₃) and 4-ethynyl-2,2-dimethyl-1,3-dioxolane48 (9%) (Found 125.0602. C₇H₉O₂ (M − H) requires 125.0603); δ_H (300 MHz) 4.72 (1 H, td, J 6.3, 2.1, 4-H), 4.18 (1 H, dd, J 8.1, 6.3, 5-H), 3.95 (1 H, dd, J 8.1, 6.3, 5-H), 2.51 (1 H, d, J 2.1, ≡CH), 1.50 (3 H, q, J 0.6) and 1.39 (3 H, q, J 0.6); δ_C (75 MHz) 110.5 (dioxolane C-2), 85.4 (–C≡), 73.9 (HC≡), 69.8 (dioxolane C-5), 65.2 (dioxolane C-4), 26.1 and 25.8 (CMe₂).

FVP of ylide 17 (120 mg) at 650 °C gave at the furnace exit Ph₃PO; δ_P +29.2 and in the cold trap a mixture of 47 (39%) and 48 (30%), data as above.

Conclusions

Our results show that the tetrahydrofuran, tetrahydropyran and 2,2-dimethyl-1,3-dioxolane rings are generally stable under the conditions required for alkyne formation using the FVP of stabilised phosphonium ylides. Increasing the pyrolysis temperature does lead to observation of new, largely carbene-based, processes in some cases but provided the temperature is carefully controlled the use of this method to construct acetylenic carbohydrate derivatives should be possible and further results on this will be reported shortly.

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Footnotes

† Electronic supplementary information (ESI) available: Copies of ¹H and ¹³C NMR spectra of all compounds. CCDC 1435297–1435299. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ob02467f

‡ Present address: School of Biology, Chemistry and Forensic Science, University of Wolverhampton, Wulfruna Street, Wolverhampton, WV1 1LY, UK. E-mail: nazira.karodia@wlv.ac.uk

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