Configuration-guided reactions: the case of highly decorated spiro[cyclopropane-1,2′(3′H)-pyrrolo[1,2-b]isoxazole] derivatives en route to polyhydroxyindolizidines

Abstract

In the course of studies directed toward the synthesis of 7,8-disubstituted 1,2-dihydroxyindolizidines as analogues of the proapoptotic iminosugar lentiginosine, marked reactivity differences of diastereomeric spiro[cyclopropane-1,2′(3H)-pyrrolo[1,2-b]isoxazolidines] precursors were observed. While one minor diastereoisomer gives the Brandi–Guarna rearrangement to indolizidinone very smoothly, and also easily undergoes fluorination of the primary alcohol, the main diastereoisomer behaves in a rather different way. The thermal rearrangement is less efficient and fluorinating agents mainly induce an unexpected cyclization to a highly strained tetracyclic derivative that, in turn, rearranges to form the uncommon 2-oxa-8-azatricyclo[6.2.1.0^4,9]undecan-5-one system with good yield. In this last process a remarkable macrocyclic dimer has been isolated that sheds light on an unprecedented different aspect of the Brandi–Guarna rearrangement.

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Introduction

(+)-Lentiginosine (1), a natural 1,2-dihydroxyindolizidine, is an inhibitor of fungal amyloglucosidases, ATPase, and chaperone activity of Heat shock protein 90 (Hsp 90).¹ The non-natural enantiomer (−)-lentiginosine (2) is a potent proapoptotic agent against tumor cells of different origin with low toxicity toward normal cells (Fig. 1).²

Fig. 1 Structure of lentiginosine enantiomers.

In the search for novel lentiginosine analogues endowed with proapoptotic properties, the synthesis of 7,8-disubstituted 1,2-dihydroxyindolizidines 3 ³ was attempted through the convenient domino thermal rearrangement (Brandi–Guarna rearrangement) of suitable 2-spiro[cyclopropane-1,2′(3H)-pyrrolo[1,2-b]isoxazolidines] 4 (Scheme 1).⁴ Unexpectedly, a marked reactivity difference of diastereomers featuring a 3-hydroxymethyl group on the convex face or on the more crowded concave face of the spiro-fused bicyclic system was found. Herein we describe the synthesis of two new polyhydroxyindolizidine analogues of (−)-lentiginosine and an uncommon 2-oxa-8-azatricyclo[6.2.1.0^4,9]undecane derivative.

Scheme 1 Retrosynthetic pathway to 7,8-disubstituted 1,2-dihydroxyindolizidines 3.

Results and discussion

1,3-Dipolar cycloaddition (1,3-DC) of dihydroxypyrroline N-oxide 5, easily derived from tartaric acid,⁵ are commonly highly stereo- and regio-selective.⁶ As nitrone 1,3-DC reactions with alkylidenecyclopropanes exclusively afford thermally stable 4-spirocyclopropane-isoxazolidines, a cyclopropylidene acetate like 6, featuring an electron-withdrawing ester group on the exo-methylene bond, was chosen as a dipolarophile to induce the correct regioselectivity.^7,4a Cycloaddition of enantiopure nitrone 5 with ethyl cyclopropylidene acetate 6 in toluene at 30 °C afforded the adducts endo–anti (3-OtBu)-7, exo–anti-8, and endo–syn-9 in a 23 : 7 : 1 ratio and 93% overall yield (Scheme 2).^8–10 The adduct configuration was assigned by NOE experiments and then validated by the X-ray analysis of the reduced derivatives of 7 and 8 (see below).

Scheme 2 1,3-DC of nitrone 5 and cyclopropylidene acetate 6.

The two major adducts 7 and 8, which have the same relative configuration at the bridgehead carbon atom as lentiginosine, were selectively reduced to the corresponding 3-hydroxymethyl derivatives by diisobutylaluminum hydride (DIBAL) in high yield (Scheme 3). The relative configuration of 10 and 11 was confirmed by a single-crystal X-ray analysis of both the diastereomers (Scheme 3).

Scheme 3 Synthesis and X-ray structures of 10 and 11. Hydrogen atoms of methylene and methyl groups are omitted for clarity.

As fluorinated derivatives play an important role in bioactive compounds,¹¹ it was chosen to introduce the fluorine atom in place of the hydroxyl group in 10 and 11 in advance of carrying out the thermal rearrangement.

Dehydroxyfluorination of the primary alcohol was attempted with (diethylamino)difluorosulfonium tetrafluoroborate (XtalFluorE®)/TEA·3HF.¹² Under standard conditions, isoxazolidine 10 featuring the 3-hydroxymethyl group on the concave face of the bicyclic system afforded the unexpected cage compound 12 as the sole product in 73% yield (Scheme 4). When 10 was treated with a less reactive reagent such as perfluoro-1-butanesulfonyl fluoride/1,8-diazabicyclo[5.4.0]undec-7-ene (PBSF/DBU),¹³ the tetracyclic compound 12 was formed in 38% yield along with the 3-fluoromethyl and 3-methylene derivatives 13 and 14 in 10% and 41% yield, respectively. Epimer 11 afforded the corresponding fluoro derivative 15 in low yield (10%) by treatment with (XtalFluorE®)/TEA·3HF, but with a satisfactory 55% yield with PBSF/DBU (Scheme 4).

Scheme 4 Reaction of diastereomers 10 and 11 with XtalFluorE®/TEA·3HF and PBSF/DBU.

The structure of the unexpected 2,6-dioxa-7-azatricyclo[5.2.1.0^4,8]decane ring system was confirmed by a single-crystal X-ray analysis of 12 (Scheme 5). Its formation can be rationalized by an intramolecular S_N2 reaction where the C-5 tert-butoxy group acts as a nucleophile on the activated primary alcohol in intermediate 16. This process is favored by the close spatial proximity between the two reactive centers as shown by a distance of only 3.4 Å between O-5 and carbinol carbon in the crystal structure of the precursor 10. Analogous fluorinating agent-induced debenzylative cycloetherization is precedented¹⁴ but no examples involving a tert-butylether as a nucleophile have been reported up to now.

Scheme 5 Possible mechanism accounting for the formation of 12 and X-ray structure. Hydrogen atoms of methylene and methyl groups are omitted for clarity.

The next step was the formation of the indolizidine bicyclic system through the Brandi–Guarna thermal rearrangement of 5-spirocyclopropaneisoxazolidines.¹⁵ Although this rearrangement reaction has been extensively investigated and applied to the synthesis of various azaheterocycles,⁴ to date there are no examples of rearrangement of spiro[cyclopropane-1,2′(3H)-pyrrolo[1,2-b]isoxazolidines] monosubstituted on C-3 with a group other than an alkoxycarbonyl moiety.

The rearrangement was carried out in toluene at 170 °C under MW heating (Scheme 6). We were surprised to find a significant difference in rearrangement efficiency between diastereomers 10 and 11. The yield for indolizidinone 17 was significantly lower than that of 18 (25 vs. 71% yield), also highlighting the importance of relative spatial orientation of the substituent on C-3 in this process (Scheme 6).

Scheme 6 Thermal rearrangement of diastereomers 10 and 11.

Furthermore, enaminone 19 was obtained in place of the expected by-product 22.⁴ Likely, β-hydroxyketone 21 formed by a 1,5-H shift from diradical intermediate 20, undergoes a retro-aldol reaction with the extrusion of formaldehyde under the reaction conditions (Scheme 7).

Scheme 7 Proposed mechanism for the formation of enaminone 22.

Under the same rearrangement conditions, fluoro derivative 15 affords indolizidinone 23 in 45% yield along with the product of β-elimination 24 whose formation was observed by NMR analysis of the crude reaction mixture (Scheme 8). Likely, ketone 23 is more prone to undergo HF elimination than 15 under the reaction conditions, accordingly, 24 should derive directly from 23 and the overall yield of the thermal rearrangement of fluoro derivative 15 is good (70%) and comparable to that of the corresponding alcohol 11 (71%). With substrate 15 no formation of enaminone by-products was observed. Enone 24 could not be isolated because it undergoes dimerization through hetero-Diels–Alder cycloaddition under high concentration conditions and during chromatography on silica gel. Compound 23 slowly evolves to form the dimer of 24 when stored in a fridge, supporting the hypothesis of the propensity of 23 to eliminate HF (Scheme 8).

Scheme 8 Thermal rearrangement of fluoro derivative 15.

The thermal rearrangement of cage compound 12 was also attempted under standard conditions. The expected 2-oxa-8-azatricyclo[6.2.1.0^4,9]undecane-5-one 25 was formed in good 55% yield (Scheme 9). Surprisingly, product 26 featuring the double molecular weight of 25 was also isolated in 22% yield. The structures of both 25 and 26 were unambiguously established by single-crystal X-ray analysis (Scheme 9).

Scheme 9 Thermal rearrangement of 12 and X-ray structures of 25 and 26. Hydrogen atoms of methylene and methyl groups are omitted for clarity.

The formation of 26 is an interesting novelty that provides new insights into the Brandi–Guarna rearrangement.^4,16 In fact, a macrocyclic dimer such as 26 was never observed in the previous studies of this reaction. We hypothesized that the bicyclic diradical intermediate 27, originated from 12 through the common N–O homolysis of the isoxazolidine ring followed by cyclopropyl opening and C=O double bond formation, undergoes a hydrogen-atom abstraction from the carbonyl Cα by the nitrogen-centered radical to give the α,β-unsaturated ketone 29 (Scheme 10). The most common 1,5-H shift occurring in these rearrangements, for example the one leading to compound 21 (Scheme 7), in the rigid intermediate 27 is likely hampered by the lower accessibility of the bridge-head hydrogen α to nitrogen and would lead to a rather strained pyrroline 28. Enone 29, then, can undergo intramolecular Michael addition to pyridinone 25 ¹⁷ or intermolecular head-to-tail cyclization to [1,7]diazacyclododecinedione derivative 26. The presumed intermediate 29, and any open chain dimer, were not observed in the reaction mixture, therefore their formation is only postulated. On the other hand, a double head-to-tail radical coupling of 27, albeit possible, sounds very unlikely due to the short life time of a diradical species such as 27. In addition, a similar behavior has not been observed previously, suggesting a different path of 1,5-H shift for 27.

Scheme 10 Proposed mechanism of the thermal rearrangement of 12.

Reduction with NaBH₄ of cyclic ketones 18, 23, and 25 afforded the corresponding alcohols 30, 32, and 33 in good yield (67–84%) and high diastereoselectivity (Scheme 11). In particular, the attack of the hydride occurred preferentially on the convex face (on the same side of the bridgehead protons) of either the tricyclic or bicyclic systems. Epimeric alcohols were formed in minor amounts (≤10% yield). Deprotection of the tert-butoxy group by treatment with TFA completed the synthesis of lentiginosine analogues 31, 34, and 35 (Scheme 11).

Scheme 11 Synthesis of lentiginosine analogues 31, 34, and 35.

Conclusions

The effect on the reactivity of a hydroxymethyl group at C-3 of spiro[cyclopropane-1,2′(3H)-pyrrolo[1,2-b]isoxazolidines] was examined. When this moiety is in the convex face of the spiro-fused bicyclic system it can undergo standard dehydroxyfluorination and thermal rearrangement reactions. In particular, the rearrangement to 8-substituted indolizidinones appears to be favored by the presence of a C-3 alkyl substituent as both the hydroxyl and fluoro derivatives 11 and 15 gave bicyclic rearrangement products in higher yield with respect to C-3 unsubstituted analogues.⁴ In contrast, a 3-hydroxymethyl group oriented into the more crowded bicyclic concave face partially hampers the thermal rearrangement lowering the indolizidinone yield. Furthermore, activation of the hydroxy group in 10 with fluorinating agents is preferentially followed by the intramolecular attack of the spatially close 6-tert-butoxy group instead of the intermolecular substitution with a fluoride anion. This process allowed the synthesis in good yield of an uncommon hetero-polycyclic system such as 2,6-dioxa-7-azaspiro[tricyclo[5.2.1.0^4,8]decane-5,1′-cyclopropane] 12 and 2-oxa-8-azatricyclo[6.2.1.0^4,9]undecane-5-one 25 in turn obtained by the thermal rearrangement of 12 along with a previously never observed macrocyclic byproduct 26. The main indolizidinone derivatives 18, 23, and 25 were then converted into lentiginosine analogues 31, 34, and 35.

Biological activities of 31, 34, and 35 and the synthesis of C-8 epimers of 34 and 35 through a different approach are under investigation and will be reported in due course.

Experimental

General

Microwave-assisted reactions were carried out in a CEM Discover™ single-mode microwave reactor with an IR temperature sensor. R_f values refer to TLC on 0.25 mm silica gel plates. Melting points (m.p.) were determined on a Thiele Electrothermal apparatus and on a Bibby–Stuart Scientific apparatus SMP3. Polarimetric measurements were performed on a JASCO DIP-370. NMR spectra were measured on Varian Gemini (¹H, 200 MHz,¹³C, 50 MHz), Varian Mercury (¹H, 400 MHz, ¹³C, 100 MHz) and Varian INOVA (¹H, 400 MHz, ¹³C, 100 MHz) nuclear magnetic resonance spectrometers; CDCl₃ was used as the solvent in NMR analyses. NMR data are reported in δ (ppm) from TMS at 25 °C and peak assignments were made on the basis of ¹H–¹H COSY, HSQC and HMBC experiments. IR spectra were recorded with a Perkin-Elmer Spectrum BX FT-IR System spectrophotometer on CDCl₃ solutions. Elemental analyses were performed with a Perkin-Elmer 2400 analyzer and CHN-S Thermo Finnigan Analyzer. MS (ESI) were recorded on a LCQ Fleet Ion Trap Mass Spectrometer with a Surveyor Plus LC System (Thermo Scientific) operating in positive (⁺ESI) and negative (⁻ESI) ion mode by direct infusion of a methanolic solution of the sample. Accurate mass spectra were recorded on a LTQ-Orbitrap high-resolution mass spectrometer (Thermo, San Jose, CA, USA), equipped with a conventional ESI source.

1,3-Dipolar cycloaddition. A mixture of nitrone 5 (1 g, 4.36 mmol) and ethyl cyclopropylideneacetate 6 (715 mg, 5.67 mmol) in toluene (2.9 mL) was heated in a microwave reactor at 30 °C for 75 min. The reaction mixture was diluted with MeOH and then evaporated under reduced pressure. The crude residue was purified by chromatography on silica gel (gradient elution with petroleum ether/Et₂O = 1.5 : 1 to Et₂O) to afford adducts 7 (1.09 g, 70%), 8 (309 mg, 20%), and 9 (39 mg, 3%) as pale yellow oils.

Ethyl (3′R,3a′R,4′R,5′R)-4′,5′-di-tert-butoxytetrahydro-3′H-spiro[cyclopropane-1,2′-pyrrolo[1,2-b]isoxazole]-3′-carboxylate (7). R _f = 0.30 (petroleum ether/Et₂O = 1 : 1). [α]²⁴_D = −135.8 (c = 0.97, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.27 (the A part of an ABX₃ system, J = 10.8, 7.2 Hz, 1H, CHHMe), 4.21 (the B part of an ABX₃ system, J = 10.8, 7.2 Hz; 1H, CHHMe), 4.17 (dd, J = 6.5, 4.9 Hz, 1H, 4′-H), 4.03 (pseudo dt, J = 10.3, 6.3 Hz, 1H, 5′-H), 3.91 (dd, J = 8.4, 4.9 Hz, 1H, 3a′-H), 3.38 (dd, J = 8.5, 6.1 Hz, 1H, 6′-Ha), 3.30 (dd, J = 10.3, 8.5 Hz, 1H, 6′-Hb), 3.14 (d, J = 8.4 Hz, 1H, 3′-H), 1.31 (t, J = 7.2 Hz, 3H, CH₂CH₃), 1.16 [s, 9H, C(CH₃)₃], 1.14 [s, 9H, C(CH₃)₃], 1.15–1.04 (m, 1H, c-Pr), 1.01–0.89 (m, 2H, c-Pr), 0.64–0.53 (m, 1H, c-Pr) ppm. ¹³C NMR (CDCl₃, 50 MHz): δ = 170.9 (s; C=O), 76.7 (d; C-5′), 75.9 (d; C-4′), 73.8 [s; C(CH₃)₃], 73.5 [s; C(CH₃)₃], 72.7 (d; C-3a′), 66.0 (s; C-2′), 60.9 (t; CH₂CH₃), 59.2 (t; C-6′), 53.3 (d; C-3′), 28.6 [q; 3C, C(CH₃)₃], 28.5 [q; 3C, C(CH₃)₃], 14.2 (q; CH₂CH₃), 12.2 (t; c-Pr), 7.9 (t; c-Pr) ppm. IR (CDCl₃): v = 2978, 2935, 1732, 1472, 1462, 1365, 1192, 1094 cm⁻¹. MS (ESI) m/z = 356 [M + H]⁺; 378 [M + Na]⁺. C₁₉H₃₃NO₅ (355.47): calcd C, 64.20; H, 9.36; found C, 64.32; H, 8.97.

Ethyl (3′S,3a′R,4′R,5′R)-4′,5′-di-tert-butoxytetrahydro-3′H-spiro[cyclopropane-1,2′-pyrrolo[1,2-b]isoxazole]-3′-carboxylate (8). R _f = 0.39 (petroleum ether/Et₂O = 1 : 1). [α]²⁴_D = +8.29 (c = 0.86, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.18 (the A part of an ABX₃ system, J = 10.8, 7.1 Hz, 1H, CHHMe), 4.16 (the B part of an ABX₃ system, J = 10.8, 7.1 Hz; 1H, CHHMe), 4.03 (dd, J = 5.6, 3.3 Hz, 1H, 3a′-H), 3.98–3.91 (m, 2H, 5′-H + 4′-H), 3.44 (dd, J = 10.0, 5.5 Hz, 1H, 6′-Ha), 3.26 (d, J = 5.6 Hz, 1H, 3′-H), 3.19–3.12 (m, 1H, 6′-Hb), 1.25 (t, J = 7.1 Hz, 3H, CH₂CH₃), 1.18 [s, 9H, C(CH₃)₃], 1.16 [s, 9H, C(CH₃)₃], 1.24–1.06 (m, 1H, c-Pr), 0.96–0.82 (m, 2H, c-Pr), 0.77–0.66 (m, 1H, c-Pr) ppm. ¹³C NMR (CDCl₃, 50 MHz): δ = 171.5 (s; C=O), 80.8 (d; C-4′), 76.2 (d; C-5′), 74.8 (d; C-3a′), 74.03 [s; C(CH₃)₃], 74.00 [s; C(CH₃)₃], 64.8 (s; C-2′), 61.0 (t; CH₂CH₃), 58.8 (t; C-6′), 56.8 (d; C-3′), 28.6 [q; 3C, C(CH₃)₃], 28.4 [q; 3C, C(CH₃)₃], 14.5 (t; c-Pr), 14.2 (q; CH₂CH₃), 4.3 (t; c-Pr) ppm. IR (CDCl₃): v = 2978, 2936, 2872, 1734, 1473, 1462, 1366, 1189, 1110 cm⁻¹. MS (ESI) m/z = 356 [M + H]⁺; 378 [M + Na]⁺. C₁₉H₃₃NO₅ (355.47): calcd C, 64.20; H, 9.36; found C, 63.89; H, 9.40.

Ethyl (3′S,3a′S,4′R,5′R)-4′,5′-di-tert-butoxytetrahydro-3′H-spiro[cyclopropane-1,2′-pyrrolo[1,2-b]isoxazole]-3′-carboxylate (9). R _f = 0.11 (petroleum ether/Et₂O = 1 : 1). [α]²⁵_D = −3.22 (c = 0.92, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.55 (pseudo dt, J = 8.7, 7.6 Hz, 1H, 5′-H), 4.27 (pseudo t, J = 8.8 Hz, 1H, 3a′-H), 4.16 (the A part of an ABX₃ system, J = 10.9, 7.2 Hz, 1H, CHHMe), 4.12 (the B part of an ABX₃ system, J = 10.9, 7.2 Hz; 1H, CHHMe), 4.04 (pseudo t, J = 8.0 Hz, 1H, 4′-H), 3.49 (dd, J = 13.8, 7.4 Hz, 1H, 6′-Ha), 2.96 (d, J = 9.3 Hz, 1H, 3′-H), 2.88 (dd, J = 13.8, 8.7 Hz, 1H, 6′-Hb), 1.28 (t, J = 7.2 Hz, 3H, CH₂CH₃), 1.15 [s, 9H, C(CH₃)₃], 1.14 [s, 9H, C(CH₃)₃], 1.14–1.02 (m, 1H, c-Pr), 0.98–0.80 (m, 2H, c-Pr), 0.63–0.54 (m, 1H, c-Pr) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 172.2 (s; C=O), 77.2 (d; C-4′), 74.2 [s; 2C, C(CH₃)₃], 73.5 (d; C-5′), 70.2 (d; C-3a′), 64.6 (s; C-2′), 60.5 (t; CH₂CH₃), 59.1 (t; C-6′), 53.9 (d; C-3′), 28.5 [q; 3C, C(CH₃)₃], 28.0 [q; 3C, C(CH₃)₃], 14.2 (q; CH₂CH₃), 12.9 (t; c-Pr), 6.7 (t; c-Pr) ppm. IR (CDCl₃): v = 2977, 2932, 2872, 1732, 1462, 1365, 1194, 1120 cm⁻¹. MS (ESI) m/z = 356 [M + H]⁺; 378 [M + Na]⁺. C₁₉H₃₃NO₅ (355.47): calcd C, 64.20; H, 9.36; N, 3.94; found C, 63.83; H, 9.43; N, 3.75.

Reduction of 7. A solution of DIBAL-H in toluene (1 M, 9.7 mL, 9.7 mmol) was added dropwise to a solution of adduct 7 (980 mg, 2.76 mmol) in anhydrous CH₂Cl₂ (9.2 mL) cooled at 0 °C and under an N₂ atmosphere. The reaction mixture was stirred at 0 °C for 1 h and then treated with MeOH (20 mL), filtered on a short pad of Celite and concentrated under reduced pressure. The crude residue was diluted with CH₂Cl₂ and the solution was dried on anhydrous Na₂SO₄, filtered and concentrated under reduced pressure. Purification by chromatography on silica gel (elution with CH₂Cl₂/MeOH = 25 : 1) gave alcohol 10 (745 mg, 86%) as a white solid.

[(3′S,3a′R,4′R,5′R)-4′,5′-Di-tert-butoxytetrahydro-3′H-spiro[cyclopropane-1,2′-pyrrolo[1,2-b]isoxazol]-3′-yl]methanol (10). 0.25 (CH₂Cl₂/MeOH = 25 : 1). M.p. = 111–113 °C (Et₂O). [α]²³_D = −133.6 (c = 0.84, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.21 (pseudo t, J = 3.2 Hz, 1H, 4′-H), 4.02 (ddd, J = 5.4, 4.4, 3.2 Hz, 1H, 5′-H), 3.89 (dd, J = 9.3, 3.6 Hz, 1H, 3a′-H), 3.83–3.77 (m, 2H, CH₂OH), 3.56–3.48 (m, 1H, OH), 3.42 (dd, J = 13.1, 5.4 Hz, 1H, 6′-Ha), 3.21 (dd, J = 13.1, 4.4 Hz, 1H, 6′-Hb), 2.69 (pseudo dt, J = 9.3, 5.0 Hz, 1H, 3′-H), 1.22 [s, 9H, C(CH₃)₃], 1.20 [s, 9H, C(CH₃)₃], 1.08–0.98 (m, 1H, c-Pr), 0.93–0.76 (m, 2H, c-Pr), 0.55–0.43 (m, 1H, c-Pr) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 79.4 (d; C-5′), 78.6 (d; C-4′), 75.4 (d; C-3a′), 74.8 [s; C(CH₃)₃], 74.6 [s; C(CH₃)₃], 63.5 (s; C-2′), 61.1 (t; C-6′), 61.0 (t; CH₂-OH), 49.0 (d; C-3), 28.6 [q; 3C, C(CH₃)₃], 28.3 [q; 3C, C(CH₃)₃], 10.6 (t; c-Pr), 7.2 (t; c-Pr) ppm. IR (CDCl₃): v = 3622, 3365, 2977, 2936,1391, 1366, 1188, 1076 cm⁻¹. MS (ESI) m/z = 314 [M + H]⁺; 336 [M + Na]⁺. C₁₇H₃₁NO₄ (313.43): calcd C, 65.14; H, 9.97; found C, 64.79; H, 9.60.

Reduction of 8. Following the same procedure used to prepare 10, diasteromeric alcohol 11 was obtained starting from 8 (252 mg, 0.71 mmol). Purification by chromatography on silica gel (elution with CH₂Cl₂/MeOH = 25 : 1) gave 11 (200 mg, 90%) as a white solid.

[(3′R,3a′R,4′R,5′R)-4′,5′-Di-tert-butoxytetrahydro-3′H-spiro[cyclopropane-1,2′-pyrrolo[1,2-b]isoxazol]-3′-yl]methanol (11). R _f = 0.26 (CH₂Cl₂/MeOH = 20 : 1). M.p. = 135–137 °C (dec., Et₂O). [α]²²_D = +9.45 (c = 0.51, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 3.98 (pseudo t, J = 5.4 Hz, 1H, 4′-H), 3.89 (pseudo dt, J = 8.4, 5.9 Hz, 1H, 5′-H), 3.75–3.66 (m, 2H, CH₂OH), 3.53–3.45 (m, 2H, 3a′-H + 6′-Ha), 3.18 (dd, J = 10.1, 8.8 Hz, 1H, 6′-Hb), 2.51–2.46 (m, 1H, 3′-H), 2.16 (br s, 1H, OH), 1.20 [s, 9H, C(CH₃)₃], 1.18 [s, 9H, C(CH₃)₃], 1.10–1.00 (m, 1H, c-Pr), 0.89–0.71 (m, 3H, c-Pr) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 80.7 (d; C-4′), 75.7 (d; C-5′), 74.03 [s; C(CH₃)₃], 73.96 [s; C(CH₃)₃], 73.4 (d; C-3a′), 64.0 (t; CH₂-OH), 63.9 (s; C-2′), 59.3 (t; C-6′), 52.5 (d; C-3′), 29.0 [q; 3C, C(CH₃)₃], 28.5 [q; 3C, C(CH₃)₃], 13.4 (t; c-Pr), 1.9 (t; c-Pr) ppm. IR (CDCl₃): v = 3626, 2977, 2935, 2878, 1390, 1365, 1191, 1105 cm⁻¹. MS (ESI) m/z = 314 [M + H]⁺; 336 [M + Na]⁺. C₁₇H₃₁NO₄ (313.43): calcd C, 65.14; H, 9.97; N, 4.47; found C, 64.81; H, 9.99; N, 4.28.

Reaction of alcohol 10 with (XtalFluorE®)/TEA·3HF. TEA·3HF (0.066 mL, 0.407 mmol) and XtalFluorE® (75 mg, 0.327 mmol) were sequentially added to a solution of 10 (51 mg, 0.163 mmol) in anhydrous CH₂Cl₂ (0.6 mL) cooled at −78 °C. The reaction mixture was slowly heated to r.t. (3.5 h) and then stirred at r.t. for 1 h. The solvent was removed under reduced pressure and the residue was purified by chromatography on silica gel (elution with EtOAc/petroleum ether = 1 : 1) to afford 12 (28.6 mg, 73%) as a white solid.

(3a′S,6′R,7′R,7a′R)-7′-tert-Butoxytetrahydro-4′H-spiro[cyclopropane-1,3′-[2,5]dioxa[1]aza[1,6]methanopyrano[4,3-c]isoxazole] (12). R _f = 0.27 (EtOAc/petroleum ether = 1 : 1). M.p. = 99–101 °C (i-Pr₂O). [α]²⁴_D = −14.08 (c = 0.520, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.49–4.46 (m, 1H, 6′-H), 4.06–4.03 (m, 1H, 7′-H), 4.02 (br d, J = 8.1 Hz, 1H, 7a′-H), 3.77 (dd, J = 11.9, 4.2 Hz, 1H, 4′-Ha), 3.56 (dd, J = 14.2, 3.3 Hz, 1H, 8′-Ha), 3.36 (dd, J = 11.9, 1.5 Hz, 1H, 4′-Hb), 3.07 (br d, J = 14.2 Hz, 1H, 8′-Hb), 2.67 (ddd, J = 8.1, 4.2, 1.5 Hz, 1H, 3a′-H), 1.23 [s, 9H, C(CH₃)₃], 1.04–0.89 (m, 2H, c-Pr), 0.87–0.79 (m, 1H, c-Pr), 0.56–0.50 (m, 1H, c-Pr) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 77.5 (d; C-7′), 74.9 [s, C(CH₃)₃], 71.1 (d, C-6′), 70.8 (d, C-7a′), 67.1 (s, C-3′), 61.7 (t, C-8′), 59.3 (t, C-4′), 48.7 (d, C-3a′), 28.1 [q, 3C, C(CH₃)₃], 8.9 (t, c-Pr), 8.4 (t, c-Pr) ppm. IR (CDCl₃): v = 2977, 2938, 2872, 1365, 1191, 1098 cm⁻¹. MS (ESI) m/z = 240 [M + H]⁺; 262 [M + Na]⁺. C₁₃H₂₁NO₃ (239.31): calcd C, 65.25; H, 8.84; N, 5.85; found C, 65.31; H, 9.04; N, 5.60.

Reaction of alcohol 10 with PBSF/DBU. PBSF (0.07 mL, 0.403 mmol) was added dropwise to a solution of 10 (50 mg, 0.159 mmol) and DBU (0.055 mL, 0.403 mmol) in anhydrous THF (1.3 mL). The reaction mixture was stirred at 70 °C for 15 h and then the solvent was removed under reduced pressure. Purification of the residue by chromatography on silica gel (elution with petroleum ether/EtOAc = 2.5 : 1) gave 14 (19 mg, 41%) as a white solid along with 12 (14 mg, 38%), and 13 (5 mg, 10%).

(3a′R,4′R,5′R)-4′,5′-Di-tert-butoxy-3′-methylenetetrahydro-3′H-spiro[cyclopropane-1,2′-pyrrolo[1,2-b]isoxazole] (14). R _f = 0.4 (petroleum ether/EtOAc = 3 : 1). M.p. = 56.7–57.6 °C. [α]²⁵_D = −87.24 (c = 0.58, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.93–4.90 (m, 1H, =CHH), 4.61–4.58 (m, 1H, =CHH), 4.12–4.07 (m, 1H, 3a′-H), 4.03–3.94 (m, 2H, 4′-H + 5′-H), 3.40 (dd, J = 10.1, 6.2 Hz, 1H, 6′-Ha), 2.99 (pseudo t, J = 9.5 Hz, 1H, 6′-Hb), 1.23 [s, 9H, C(CH₃)₃], 1.16 [s, 9H, C(CH₃)₃], 1.25–1.09 (m, 2H, c-Pr), 0.91–0.75 (m, 2H, c-Pr) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 153.8 (s, C-3′), 100.0 (t, =CH₂), 81.8 (d; C-4′), 77.6 (d; C-5′), 75.0 (d, C-3a′), 74.3 [s, C(CH₃)₃], 73.9 [s, C(CH₃)₃], 63.5 (s, C-2′), 58.7 (t, C-6′), 28.8 [q, 3C, C(CH₃)₃], 28.5 [q, 3C, C(CH₃)₃], 17.9 (t, c-Pr), 11.6 (t, c-Pr) ppm. IR (CDCl₃): v = 2979, 2935, 2902, 1814, 1793, 1472, 1382, 1190, 1097 cm⁻¹. MS (ESI) m/z = 296 [M + H]⁺; 318 [M + Na]⁺. C₁₇H₂₉NO₃ (295.42): calcd C, 69.12; H, 9.89; N, 4.74; found C, 69.45; H, 9.50; N, 4.37.

(3′R,3a′R,4′R,5′R)-4′,5′-Di-tert-butoxy-3′-(fluoromethyl)tetrahydro-3′H-spiro[cyclopropane-1,2′-pyrrolo[1,2-b]isoxazole] (13). R _f = 0.31 (petroleum ether/EtOAc = 3 : 1). [α]²⁶_D = −88.91 (c = 0.550, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.82 (ddd, J_F = 46.6; J = 9.4, 7.6 Hz, 1H, CHHF), 4.59 (ddd, J_F = 47.2; J = 9.4, 6.7 Hz, 1H, CHHF), 4.08 (pseudo t, J = 3.3 Hz, 1H, 4′-H), 3.78 (pseudo dt, J = 3.6, 5.9 Hz, 1H, 5′-H), 3.91 (dd, J = 8.9, 3.0 Hz, 1H, 3a′-H), 3.44 (dd, J = 11.7, 5.6 Hz, 1H, 6′-Ha), 3.05 (dd, J = 11.7, 6.1 Hz, 1H, 6′-Hb), 2.81 (pseudo ddt, J_F = 16.1; J = 9.1, 7.2 Hz, 1H, 3′-H), 1.20 [s, 9H, C(CH₃)₃], 1.18 [s, 9H, C(CH₃)₃], 1.00–0.81 (m, 3H, c-Pr), 0.57–0.49 (m, 1H, c-Pr) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 83.6 (dt, CH₂-F, J_F = 166.8), 78.3 (d, C-5′), 77.0 (d, C-4′), 74.5 (dd, C-3a′, J_F = 2.8), 74.2 [s, C(CH₃)₃], 74.0 [s, C(CH₃)₃], 64.0 (d, C-2′, J_F = 8.9), 61.0 (t, C-6′), 46.6 (dd, C-3, J_F = 19.4), 28.5 [q, 3C, C(CH₃)₃], 28.4 [q, 3C, C(CH₃)₃], 9.3 (t, c-Pr), 7.5 (t, c-Pr) ppm. IR (CDCl₃): v = 2978, 2933, 2002, 2872, 1793, 1472, 1382, 1190, 1097, cm⁻¹. MS (ESI) m/z = 316 [M + H]⁺; 338 [M + Na]⁺; 631 [2M + Na]⁺. C₁₇H₃₀FNO₃ (315.42): calcd C, 64.73; H, 9.59; N, 4.44; found C, 64.36; H, 9.51; N, 4.08.

Reaction of alcohol 10 with PBSF/DBU. PBSF (0.07 mL, 0.403 mmol) was added dropwise to a mixture of 11 (50 mg, 0.159 mmol) and DBU (61 mg, 0.403 mmol) in anhydrous THF (1.3 mL). The reaction mixture was stirred at r.t for 30 min and then the solvent was removed under reduced pressure. Purification of the crude mixture by chromatography on silica gel (elution with petroleum ether/EtOAc = 3 : 1) gave 15 (28 mg, 55%) and 14 (1 mg, 2%) as white solids.

(3′S,3a′R,4′R,5′R)-4′,5′-Di-tert-butoxy-3′-(fluoromethyl)tetrahydro-3′H-spiro[cyclopropane-1,2′-pyrrolo[1,2-b]isoxazole] (15). R _f = 0.31 (petroleum ether/EtOAc = 3 : 1). M.p. = 43–44 °C. [α]²⁴_D = +11.59 (c = 0.635, CHCl₃) ¹H NMR (CDCl₃, 400 MHz): δ = 4.51 (ddd, J_F = 41.8; J = 9.1, 7.6 Hz, 1H, CHHF), 4.39 (ddd, J_F = 40.7; J = 9.1, 6.8 Hz, 1H, CHHF), 3.97 (pseudo t, J = 5.1 Hz, 1H, 4′-H), 3.89 (pseudo dt, J = 8.3, 5.8 Hz, 1H, 5′-H), 3.48 (dd, J = 10.3, 5.9 Hz, 1H, 6′-Ha), 3.36 (pseudo t, J = 4.2 Hz, 1H, 3a′-H), 3.19 (dd, J = 10.3, 8.4 Hz, 1H, 6′-Hb), 2.70 (pseudo ddt, J_F = 14.0; J = 3.9, 7.1 Hz, 1H, 3′-H), 1.20 [s, 9H, C(CH₃)₃], 1.18 [s, 9H, C(CH₃)₃], 1.08–1.00 (m, 1H, c-Pr), 0.85–0.72 (m, 3H, c-Pr) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 84.0 (dt, CH₂-F, J_F = 171.0), 80.7 (d, C-4′), 75.8 (d, C-5′), 74.1 [s, C(CH₃)₃], 74.0 [s, C(CH₃)₃], 73.0 (dd, C-3a′, J_F = 5.5), 63.1 (d, C-2′, J_F = 6.6), 59.2 (t, C-6′), 51.1 (dd, C-3, J_F = 18.9), 28.9 [q, 3C, C(CH₃)₃], 28.5 [q, 3C, C(CH₃)₃], 13.1 (t, c-Pr), 1.7 (t, c-Pr) ppm. IR (CDCl₃): v = 2977, 2934, 2873, 1391, 1366, 1190, 1099 cm⁻¹. MS (ESI) m/z = 316 [M + H]⁺; 338 [M + Na]⁺; 653 [2M + Na]⁺. C₁₇H₃₀FNO₃ (315.42): calcd C, 64.73; H, 9.59; N, 4.44; found C, 64.64; H, 9.24; N, 4.18.

Thermal rearrangement of 10. A solution of 10 (54 mg, 0.171 mmol) in toluene (9 mL) was heated in the microwave reactor (300 W, 170 °C) for 0.5 h and then the solvent was removed under reduced pressure. Purification by chromatography on silica gel (elution with Et₂O/petroleum ether = 2 : 1) gave 17 (14 mg, 25%) and 19 (8.8 mg, 18%) as pale yellow oils.

(1R,2R,8S,8aR)-1,2-Di-tert-butoxy-8-(hydroxymethyl)hexahydroindolizin-7(1H)-one (17). R _f = 0.29 (Et₂O/petroleum ether = 2 : 1). [α]²⁶_D = −34.27 (c = 0.4, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.21 (dd, J = 11.6, 3.5 Hz, 1H, CHHOH), 4.06 (dm, J = 5.7 Hz, 1H, 1-H), 3.86 (dm, J = 4.7 Hz, 1H, 2-H), 3.83 (ddd, J = 11.6, 3.1, 0.8 Hz, 1H, CHHOH), 3.28–3.21 (m, 1H, 5-Ha), 2.98 (d, J = 9.8 Hz, 1H, 3-Ha), 2.91–2.80 (m, 1H, 6-Ha), 2.59–2.55 (m, 1H, 8-H), 2.51 (dd, J = 5.5, 3.3 Hz, 1H, 8a-H), 2.48–2.35 (m, 3H, 3-Hb + 5-Hb + 6-Hb), 1.21 (s, 9H, CH₃ × 3), 1.17 (s, 9H, CH₃ × 3) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 210.7 (s, C-7), 79.4 (d, C-1), 78.5 (d, C-2), 74.1 (s, CMe₃), 74.0 (s, CMe₃), 72.6 (d, C-8a), 62.1 (t, CH₂OH), 60.0 (t, C-3), 52.4 (d, C-8), 50.1 (t, C-5), 40.7 (t, C-6), 28.9 (q, 3C, Me × 3), 28.7 (q, 3C, Me × 3) ppm. IR (CDCl₃): v = 3405, 2978, 2935, 2873, 1707, 1391, 1366, 1191, 1073, 1042 cm⁻¹. MS (ESI) m/z = 314 [M + H]⁺; 336 [M + Na]⁺; 346 [M + MeOH + H]⁺; 649 [2M + Na]⁺.

(1Z)-1-[(3S,4S)-3,4-Di-tert-butoxypyrrolidin-2-ylidene]butan-2-one (19). R _f = 0.45 (Et₂O/petroleum ether = 2 : 1). ¹H NMR (CDCl₃, 400 MHz): δ = 9.38 (br s, NH), 5.19 (s, 1H, HC=), 4.45 (d, J = 7.3 Hz, 1H, 3-H), 4.08 (pseudo q, J = 7.1 Hz, 1H, 4-H), 3.63 (ddd, J = 10.4, 7.2, 1.1 Hz, 1H, 5-Ha), 3.23 (dd, J = 10.4, 7.0 Hz, 1H, 5-Hb), 2.32 (q, J = 7.6 Hz, 2H, CH₂CH₃), 1.29 (s, 9H, Me × 3), 1.20 (s, 9H, Me × 3), 1.09 (t, J = 7.6 Hz, 3H, CH₂CH₃) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 200.4 (s, C=O), 165.5 (s, C-2), 88.9 (d, =CH), 78.1 (d, C-3), 75.7 (d, C-4), 75.3 (s, CMe₃), 74.2 (s, CMe₃), 50.7 (7, C-5), 35.1 (t, CH₂CH₃), 29.0 (q, 3C, Me × 3), 28.4 (q, 3C, Me × 3), 9.9 (q, CH₂CH₃) ppm. IR (CDCl₃): v = 3687, 3618, 3019, 2977, 2936, 2876, 1631, 1549, 1367, 1217, 1191, cm⁻¹. MS (ESI) m/z = 306 [M + Na]⁺; 589 [2M + Na]⁺.

Thermal rearrangement of 11. A solution of 11 (59 mg, 0.188 mmol) in toluene (9.5 mL) was heated in the microwave reactor (300 W, 150 °C) for 1 h and then the solvent was removed under reduced pressure. Purification by chromatography on silica gel (elution with EtOAc/petroleum ether = 2.5 : 1) gave 18 (41.7 mg, 71%) and 19 (2 mg, 4%) as pale yellow oils.

(1R,2R,8R,8aR)-1,2-Di-tert-butoxy-8-(hydroxymethyl)hexahydroindolizin-7(1H)-one (18). R _f = 0.25 (EtOAc/petroleum ether = 2.5 : 1). [α]²⁴_D = −38.03 (c = 0.69, CHCl₃) ¹H NMR (CDCl₃, 400 MHz): δ = 4.00–3.96 (m, 2H, CHHOH + 2-H), 3.93 (dd, J = 5.2, 2.3 Hz, 1H, 1-H), 3.79 (dd, J = 11.9, 6.6 Hz, 1H, CHHOH), 3.21 (ddd, J = 11.7, 6.5, 1.7 Hz, 1H, 5-Ha), 3.03 (dd, J = 10.1, 2.4 Hz, 1H, 3-Ha), 2.93–2.76 (br s, OH), 2.79 (dd, J = 10.1, 6.6 Hz, 1H, 3-Hb), 2.74–2.54 (m, 3H, 5-Hb + 6-Ha + 8-H), 2.45 (dd, J = 10.6, 5.3 Hz, 1H, 8a-H), 2.26 (dm, J = 14.1 Hz, 1H, 6-Hb), 1.23 (s, 9H, CH₃ × 3), 1.19 (s, 9H, CH₃ × 3) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 211.7 (s, C-7), 84.6 (d, C-1), 78.8 (d, C-2), 74.8 (s, CMe₃), 74.0 (s, CMe₃), 68.4 (d, C-8a), 59.6 (t, C-3), 59.0 (t, CH₂OH), 53.7 (d, C-8), 49.9 (t, C-5), 39.1 (t, C-6), 28.9 (q, 3C, Me × 3), 28.8 (q, 3C, Me × 3)ppm. IR (CDCl₃): v = 3567, 2978, 2935, 2874, 2803, 1697, 1392, 1366, 1189, 1070, cm⁻¹. MS (ESI) m/z = 314 [M + H]⁺; 336 [M + Na]⁺; 346 [M + MeOH + H]⁺. C₁₇H₃₁NO₄ (313.43): calcd C, 65.14; H, 9.97; N, 4.47; found C, 64.81; H, 9.59; N, 4.06.

Thermal rearrangement of 15. A solution of 15 (46 mg, 0.146 mmol) in toluene (7.3 mL) was heated in the microwave reactor (300 W, 170 °C) for 20 min and then the solvent was removed under reduced pressure. Purification by chromatography on silica gel (elution with EtOAc/petroleum ether = 1 : 1) gave 23 (21 mg, 45%) and the dimer of 24 (11 mg, 25%) as pale yellow oils.

(1R,2R,8R,8aR)-1,2-Di-tert-butoxy-8-(fluoromethyl)hexahydroindolizin-7(1H)-one (23). ¹H NMR (CDCl₃, 400 MHz): δ = 4.87 (ddd, J_F = 46.5; J = 9.4, 3.5 Hz, 1H, CHHF), 4.78 (ddd, J_F = 47.0; J = 9.4, 3.9 Hz, 1H, CHHF), 4.03–3.97 (m, 2H, 1-H + 2-H), 3.29–3.20 (m, 1H, 5-Ha), 3.05–2.91 (m, 2H, 3-H), 2.90–2.62 (m, 4H, 5-Hb + 6-Ha + 8-H + 8a-H), 2.38–2.24 (m, 1H, 6-Hb), 1.23 (s, 9H, CH₃ × 3), 1.20 (s, 9H, CH₃ × 3) ppm.

Further characterization of 23 was not possible as the compound decomposed quickly after isolation. It was immediately reduced to the stable alcohol (see below for product 33).

Thermal rearrangement of 12. A solution of 12 (54 mg, 0.23 mmol) in toluene (12 mL) was heated in the microwave reactor (300 W, 136 °C) for 30 min and then the solvent was removed under reduced pressure. Purification by chromatography on silica gel (gradient elution with Et₂O to Et₂O/MeOH = 15 : 1) gave 25 (30 mg, 55%) and 26 (12.1 mg, 22%) as white solids.

(4aS,7R,8R,8aR)-8-tert-Butoxyhexahydro-4H,5H-1,7-methanopyrano[4,3-b]pyridin-4-one (25). R _f = 0.27 (Et₂O/MeOH = 15 : 1). M.p. = 101–103 °C (i-Pr₂O). [α]²⁷_D = −426.73 (c = 0.29, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.13 (br d, J = 3.1 Hz, 1H, 7-H), 3.84 (dd, J = 12.5, 8.3 Hz, 1H, 5-Ha), 3.84 (br s, 1H, 8-H), 3.59–3.48 (m, 3H, 5-Hb + 9-Ha + 2-Ha), 3.42–3.32 (m, 2H, 8a-H + 2-Hb), 2.86 (br d, J = 11.6 Hz, 1H, 9-Hb), 2.84 (dd, J = 7.9, 2.7 Hz, 1H, 4a-H), 2.70 (ddd, J = 15.2, 8.8, 6.5 Hz, 1H, 3-Ha), 2.26 (br pseudo dt, J = 15.2, 5.3 Hz, 1H, 3-Hb), 1.24 [s, 9H, C(CH₃)₃] ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 209.4 (s; C-4), 78.4 (d; C-7), 75.4 (d; C-8), 74.9 [s; C(CH₃)₃], 64.6 (d; C-8), 59.1 (t; C-5), 54.5 (t; C-9), 53.8 (d; C-4a), 49.1 (t; C-2), 35.6 (t; C-3), 28.3 [q; 3C, C(CH₃)₃] ppm. IR (CDCl₃): v = 2977, 2932, 2870, 1703, 1366, 1190, 1091 cm⁻¹. MS (ESI) m/z = 240 [M + H]⁺; 272 [M + MeOH + H]⁺. C₁₃H₂₁NO₃ (239.31): calcd C, 65.25; H, 8.84; N, 5.85; found C, 64.85; H, 8.43; N, 5.46.

(3R,4S,4aR,8aS,11R,12S,12aR,16aS)-4,12-Di-tert-butoxydodecahydro-3,5:11,13-dimethanodipyrano[4,3-b:4′,3′-h][1,7]diazacyclododecine-8,16(3H,11H)-dione (26). R _f = 0.6 (Et₂O). M.p. = 152–153 °C. [α]²⁷_D = −64.84 (c = 0.38, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.11–4.08 (m, 2H, CHOCH₂ × 2), 3.95 (pseudo t, J = 11.4 Hz, 2H, CHHCHC=O × 2), 3.65 (dd, J = 11.3, 5.7 Hz, 2H, CHHCHC=O × 2), 3.62 (br s, 2H, CHOtBu × 2), 3.54 (br s, 2H, CHN × 2), 3.32–3.25 (m, 2H, OCHCHHN × 2), 3,28 (d, J = 11.7 Hz, 2H, OCHCHHN × 2), 2.86 (dd, J = 11.7, 3.9 Hz, 2H, OCHCHHN × 2), 2.71–2.64 (m, 2H, CH₂CHHN × 2), 2.51–2.41 (m, 4H, CHHC=O × 2 + CHC=O × 2), 2.32 (br dd, J = 18.4, 6.1 Hz, 2H, CHHC=O × 2), 1.25 (s, 18H, CH₃ × 6) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 208.4 (s, 2C, C=O), 79.9 (d, 2C, CHOCH₂ × 2), 77.2 (d, 2C, CHOtBu × 2), 74.1 (s, 2C, CMe₃), 65.9 (d, 2C, CHN × 2), 60.4 (t, 2C, CH₂O × 2), 57.7 (t, 2C, CHCH₂N × 2), 54.3 (d, 2C, CHC=O × 2), 49.2 (t, 2C, CH₂CH₂N × 2), 42.9 (t, 2C, CH₂C=O × 2), 28.4 (q, 6C, CH₃ × 6) ppm. IR (CDCl₃): v = 2976, 2866, 1711, 1390, 1365, 1192, 1090 cm⁻¹. MS (ESI) m/z = 479 [M + H]⁺; 501[M + Na]⁺. C₂₆H₄₂N₂O₆ (478.62): calcd C, 65.25; H, 8.84; N, 5.85; found C, 65.64; H, 8.52; N, 5.41.

Reduction of 25. NaBH₄ (34 mg, 0.89 mmol) was added to a solution of 25 (61 mg, 0.255 mmol) in MeOH (2.7 mL) cooled at −18 °C. The reaction mixture was stirred at low temperature for 2 h and then treated with water (6 mL). MeOH was evaporated under reduced pressure and the aqueous solution was extracted with CH₂Cl₂ (5 × 5 mL). The combined organic phases were dried over anhydrous Na₂SO₄, filtered and concentrated under reduced pressure. Purification by chromatography on silica gel (gradient elution with CH₂Cl₂/MeOH = 15 : 1 to 5 : 1) afforded 30 (49 mg, 79%) and the C-7 epimer (6 mg, 10%) as white solids.

(4S,4aS,7R,8R,8aR)-8-tert-Butoxyhexahydro-2H,5H-1,7-methanopyrano[4,3-b]pyridin-4-ol (30). R _f = 0.28 (CH₂Cl₂/MeOH = 10 : 1). M.p. = 130–132 °C. [α]²⁷_D = −62.66 (c = 0.670, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.07 (d, J = 4.2 Hz, 1H, 2-H), 3.82 (pseudo dt, J = 11.2, 4.7 Hz, 1H, 7-H), 3.78–3.74 (m, 3H, 9-H + 1-H), 3.33–3.20 (m, 2H, 3-Ha + 5-Ha), 3.14–3.04 (m, 1H, 5-Hb), 2.99 (br s, 1H, 8a-H), 2.83 (d, J = 11.9 Hz, 1H, 3-Hb), 2.41–2.33 (m, 1H, 8-H), 1.91 (pseudo ddt, J = 11.6, 5.5, 12.9 Hz, 1H, 6-Ha), 1.46 (dm, J = 13.2 Hz, 1H, 6-Hb), 1.23 (s, 9H, CH₃ × 3) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 78.8 (d, C-2), 76.9 (d, C-1), 74.4 (s, CMe₃), 70.6 (d, C-7), 63.8 (d, C-8a), 58.3 (t, CH₂O), 50.8 (t, C-3), 46.7 (t, C-5), 40.0 (d, C-8), 23.2 (q, 3C, CH₃ × 3), 23.6 (t, C-6) ppm. IR (CDCl₃): v = 3607, 3352, 2977, 2937, 2865, 1392, 1365, 1241, 1193, 1087 cm⁻¹. MS (ESI) m/z = 242 [M + H]⁺; 264 [M + Na]⁺; 505 [2M + Na]⁺. C₁₃H₂₃NO₃ (241.33): calcd C, 64.70; H, 9.61; N, 5.80; found C, 64.45; H, 9.26; N, 5.61.

Reduction of 18. Following the same procedure used to prepare 30, reduction of 18 (39 mg, 0.125 mmol) afforded, after purification by chromatography on silica gel (elution with CH₂Cl₂/MeOH = 10 : 1), 32 (33 mg, 84%) as a colorless oil and the C-7 epimer (2 mg, 5%).

(1R,2R,7S,8R,8aR)-1,2-Di-tert-butoxy-8-(hydroxymethyl)octahydroindolizin-7-ol (32). R _f = 0.28 (CH₂Cl₂/MeOH = 10 : 1). [α]²⁸_D = −22.21 (c = 0.925, CHCl₃). ¹H NMR (CD₃OD, 400 MHz): δ = 4.07 (dd, J = 11.8, 2.0 Hz, 1H, CHHOH), 3.95 (dd, J = 7.6, 2.3 Hz, 1H, 1-H), 3.91–3.87 (m, 1H, 2-H), 3.80 (dd, J = 8.6, 4.0 Hz, 1H, 1-H), 2.91 (dm, J = 10.0 Hz, 1H, 3-Ha), 2.77 (ddd, J = 11.8, 5.9 Hz, 1H, CHHOH), 3.65 (pseudo dt, J = 4.8, 10.3 Hz, 1H, 7-H), 3.04 (d, J = 10.3 Hz, 1H, 3-Ha), 2.99–2.92 (m, 1H, 5-Ha), 2.45–2.35 (m, 1H, 3-Hb), 2.10–1.98 (m, 1H, 5-Hb), 1.96–1.81 (m, 2H, 6-Ha + 8a-H), 1.74 (pseudo ddt, J = 11.1, 4.5, 12.4 Hz, 1H, 6-Hb), 1.68–1.59 (m, 1H, 8-H), 1.28 (s, 9H, CH₃ × 3), 1.21 (s, 9H, CH₃ × 3) ppm. ¹³C NMR (CD₃OD, 100 MHz): δ = 84.0 (d, C-1), 78.1 (d, C-2), 75.0 (s, CMe₃), 74.0 (s, CMe₃), 71.6 (d, C-7), 66.6 (d, C-8a), 61.4 (d, CH₂OH), 60.7 (t, C-3), 50.3 (t, C-5), 48.1 (d, C-8), 33.0 (t, C-6), 29.2 (s, 3C, Me × 3), 29.0 (s, 3C, Me × 3) ppm. IR (CDCl₃): v = 3619, 3412, 2977, 2944, 2798, 1466, 1391, 1368, 1236, 1188, 1071 cm⁻¹. MS (ESI) m/z = 316 [M + H]⁺; 338 [M + Na]⁺; 653 [2M + Na]⁺. C₁₇H₃₃NO₄ (315.45): calcd C, 64.73; H, 10.54; N, 4.44; found C, 64.66; H, 10.53; N, 4.09.

Reduction of 23. Following the same procedure used to prepare 30, reduction of 23 (33 mg, 0.105 mmol) afforded, after purification by chromatography on silica gel (elution with CH₂Cl₂/MeOH = 20 : 1), 33 (22 mg, 67%) as a colorless oil and only traces of the C-7 epimer.

(1R,2R,7S,8S,8aR)-1,2-Di-tert-butoxy-8-(fluoromethyl)octahydroindolizin-7-ol (33). R _f = 0.27 (CH₂Cl₂/MeOH = 20 : 1). [α]²⁸_D = −27.50 (c = 1.00, CHCl₃). ¹H NMR (CDCl₃, 400 MHz): δ = 4.93 (ddd, J_F = 47.4; J = 9.3, 3.0 Hz, 1H, CHHF), 4.70 (ddd, J_F = 47.5; J = 9.3, 4.6 Hz, 1H, CHHF), 3.91–3.86 (m, 2H, 1-H + 2-H), 3.71–3.62 (m, 1H, 7-H), 2.98–2.89 (m, 2H, 3-Ha + 5-Ha), 2.55 (dd, J = 10.0, 6.1 Hz, 1H, 3-Hb), 2.18 (pseudo dt, J = 2.8, 12.1 Hz, 1H, 5-Hb), 2.16–2.00 (br s, OH), 2.04 (dd, J = 10.4, 6.5 Hz, 1H, 8a-H), 1.90–1.69 (m, 3H, 6-H + 8-H) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 83.8 (d, C-1), 82.8 (dt, J_F = 165.1 Hz, CH₂F), 78.4 (d, C-2), 74.5 (s, CMe₃), 73.9 (s, CMe₃), 70.3 (d, C-7), 65.2 (dd, J_F = 4.6 Hz, C-8a), 60.0 (t, C-3), 49.0 (t, C-5), 46.4 (dd, J_F = 16.5 Hz, C-8), 32.0 (t, C-6), 29.1 (q, 3C, CH₃ × 3), 29.0 (q, 3C, CH₃ × 3) ppm. IR (CDCl₃): v = 3619, 2978, 2870, 2798, 1466, 1391, 1367, 1237, 1190, 1072 cm⁻¹. MS (ESI) m/z = 318 [M + H]⁺; 340 [M + Na]⁺. HRMS (ESI): MH⁺, found 318.24352. C₁₇H₃₃FNO₃ (317.44) requires 318.24390.

Deprotection of 30. Compound 30 (44 mg, 0.182 mmol) was diluted in TFA (0.80 mL) at 0 °C. The reaction mixture was stirred at rt for 3 h and then sequentially diluted with CH₂Cl₂ and concentrated under reduced pressure. The residue was filtered through a column of Amberlyst A-26 resin (elution with MeOH) and the filtrate was concentrated under reduced pressure. Purification by chromatography on silica gel [elution with EtOAc/MeOH (5% NH₄OH) = 1 : 1] afforded 31 (30 mg, 88%) as a white waxy solid.

(4S,4aR,7R,8R,8aR)-Hexahydro-2H,5H-1,7-methanopyrano[4,3-b]pyridine-4,8-diol (31). R _f = 0.27 [EtOAc/MeOH (5% NH₄OH) = 1 : 1]. [α]²²_D = −60.51 (c = 0.590, MeOH). ¹H NMR (CDCl₃, 400 MHz): δ = 4.06 (br d, J = 4.3 Hz, 1H, 2-H), 3.85 (br s, 1H, 1-H), 3.81–3.73 (m, 2H, 7-H + CHHO), 3.68 (dd, J = 12.7, 8.7 Hz, 1H, 7-H + CHHO), 3.19 (dd, J = 12.0, 4.4 Hz, 1H, 3-Ha), 3.20–3.12 (m, 1H, 5-Ha), 3.05 (br d, J = 2.9 Hz, 1H, 8a-H), 2.98 (pseudo dt, J = 4.1, 13.9 Hz, 1H, 5-Hb), 2.89 (d, J = 12.0, 1H, 3-Hb), 2.34–2.26 (m, 1H, 8-H), 1.96 (pseudo ddt, J = 11.6, 5.4, 13.2 Hz, 1H, 6-Ha), 1.47–1.39 (m, 1H, 6-Hb) ppm. ¹³C NMR (CDCl₃, 100 MHz): δ = 79.7 (d, C-2), 77.3 (d, C-1), 71.3 (d, C-7), 64.8 (d, C-8a), 59.5 (t, CH₂O), 51.1 (t, C-3), 47.2 (t, C-5), 40.8 (t, C-8) ppm. MS (ESI) m/z = 186 [M + H]⁺. C₉H₁₅NO₃ (185.22): calcd C, 58.36; H, 8.16; N, 7.56; found C, 58.72; H, 7.79; N, 7.12.

Deprotection of 32. Following the same procedure used to prepare 31, indolizidine 34 was obtained starting from 32 (60 mg, 0.190 mmol) after 2.5 h. Purification by chromatography on silica gel [elution with EtOAc/MeOH (5% NH₄OH) = 3 : 2] gave 34 (31 mg, 81%) as a white solid.

(1R,2R,7S,8R,8aR)-8-(Hydroxymethyl)octahydroindolizine-1,2,7-triol (34). R _f = 0.28 [EtOAc/MeOH (5% NH₄OH) = 3 : 2]. M.p. = 140 °C (dec). [α]²⁵_D = +24.085 (c = 0.470, MeOH). ¹H NMR (CD₃OD, 400 MHz): δ = 4.06 (dd, J = 11.3, 4.0 Hz, 1H, CHHOH), 4.01 (ddd, J = 7.1, 3.4, 1.4 Hz, 1H, 2-H), 3.81 (dd, J = 8.0, 3.4 Hz, 1H, 1-H), 3.59 (dd, J = 11.3, 7.8 Hz, 1H, CHHOH), 3.31–3.24 (m, 1H, 7-H), 2.92 (ddd, J = 11.2, 4.4, 2.5 Hz, 1H, 5-Ha), 2.86 (dm, J = 10.5 Hz, 1H, 3-Ha), 2.53 (dd, J = 10.5, 7.1 Hz, 1H, 3-Hb), 2.06 (ddd, J = 12.6, 11.2, 2.7 Hz, 1H, 5-Hb), 1.93–1.86 (m, 1H, 6-Ha), 1.74 (dd, J = 9.9, 8.1 Hz, 1H, 8a-H), 1.70–1.55 (m, 2H, 6-Hb + 8-H) ppm. ¹³C NMR (CD₃OD, 100 MHz): δ = 84.6 (d, C-1), 78.2 (d, C-2), 73.2 (d, C-8a), 70.9 (d, C-7), 62.9 (t, CH₂OH), 62.3 (t, C-3), 51.5 (t + d, 2C, C-5 + C-8), 35.0 (t, C-6) ppm. MS (ESI) m/z = 204 [M + H]⁺; 226 [M + Na]⁺; 202 [M − H]⁻. C₉H₁₇NO₄ (203.24): calcd C, 53.19; H, 8.43; N, 6.89; found C, 53.5; H, 8.27; N, 6.63.

Deprotection of 33. Following the same procedure used to prepare 31, indolizidine 35 was obtained starting from 33 (20 mg, 0.063 mmol) after 50 min. Purification by chromatography on silica gel [elution with EtOAc/MeOH (5% NH₄OH) = 2 : 1] gave 35 (12 mg, 94%) as a white solid.

(1R,2R,7S,8S,8aR)-8-(Fluoromethyl)octahydroindolizine-1,2,7-triol (35). R _f = 0.32 [EtOAc/MeOH (5% NH₄OH) = 2 : 1]. M.p. = 121–122 °C (dec). [α]²⁴_D = +16.83 (c = 0.890, MeOH). ¹H NMR (CD₃OD, 400 MHz): δ = 4.80 (ddd, J_F = 47.0; J = 9.2, 2.1 Hz, 1H, CHHF), 4.75 (ddd, J_F = 47.3; J = 9.2, 3.3 Hz, 1H, CHHF), 3.96 (ddd, J = 6.4, 2.4, 1.0 Hz, 1H, 2-H), 3.82 (dd, J = 7.7, 2.4 Hz, 1H, 1-H), 3.54 (pseudo dt, J = 4.9, 10.7 Hz, 1H, 7-H), 2.94 (ddd, J = 11.3, 4.4, 2.4 Hz, 1H, 5-Ha), 2.84 (d, J = 10.4 Hz, 1H, 3-Ha), 2.52 (dd, J = 10.4, 6.4 Hz, 1H, 3-Hb), 2.09 (dd, J = 11.3, 2.7 Hz, 1H, 5-Hb), 1.98–1.90 (m, 2H, 6-Ha + 8a-H), 1.72–1.47 (m, 2H, 6-Hb + 8-H) ppm. ¹³C NMR (CD₃OD, 100 MHz): δ = 84.8 (d, C-1), 81.4 (dt, J_F = 166.1 Hz, CH₂F), 79.6 (d, C-2), 70.2 (d, C-8a), 68.7 (dd, J_F = 4.9 Hz, C-7), 61.9 (t, C-3), 51.4 (t, C-5), 50.6 (dd, J_F = 17.6 Hz, C-8), 35.0 (t, C-6) ppm. MS (ESI) m/z = 206 [M + H]⁺; 228 [M + Na]⁺. HRMS (ESI): MH⁺, found 206.11823. C₉H₁₇FNO₃ (205.23) requires 206.11870.

Acknowledgements

The authors thank the Italian Ministry of Education, University and Research (MIUR-Rome), for financial support (PRIN20109Z2XRJ).

Notes and references

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Footnotes

† This paper is dedicated to Professor Barry M. Trost on the occasion of his 75th birthday.

‡ Electronic supplementary information (ESI) available: Crystallographic data of compounds 10, 11, 12, 25 and 26, and NMR spectra of the new compounds. CCDC 1496575–1496579. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qo00410e

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