Synthesis of 2 , 6-trans-and 3 , 3 , 6-trisubstituted tetrahydropyran-4-ones from Maitland – Japp derived 2 H-dihydropyran-4-ones : a total synthesis of diospongin B †

6-Substituted-2H-dihydropyran-4-one products of the Maitland–Japp reaction have been converted into tetrahydropyrans containing uncommon substitution patterns. Treatment of 6-substituted-2H-dihydropyran-4-ones with carbon nucleophiles led to the formation of tetrahydropyran rings with the 2,6-transstereochemical arrangement. Reaction of the same 6-substituted-2H-dihydropyran-4-ones with L-Selectride led to the formation of 3,6-disubstituted tetrahydropyran rings, while trapping of the intermediate enolate with carbon electrophiles in turn led to the formation 3,3,6-trisubstituted tetrahydropyran rings. The relative stereochemical configuration of the new substituents was controlled by the stereoelectronic preference for pseudo-axial addition of the nucleophile and trapping of the enolate from the opposite face. Application of these methods led to a synthesis of the potent anti-osteoporotic diarylheptanoid natural product diospongin B.


Introduction
Substituted tetrahydropyran (THP) rings are present in a large number of biologically active natural products, and as such their synthesis has received much attention over the years. 1,2 On inspection of these THP rings it is clear that some substitution patterns occur more often than others, and this has resulted in a greater amount of synthetic effort being directed towards their synthesis compared to the synthesis of other substitution patterns. The consequences of those efforts are that these common substitution patterns can now be accessed readily, while the more uncommon substitution patterns still require greater synthetic effort. For example, 2,6-cis-THP rings can be accessed by a wide variety of methods, including thermodynamically controlled oxy-Michael reactions, 3,4 Diels-Alder reactions, 5 Prins rearrangements, 6,7 reduction of cyclic oxocarbenium ions, 8 metal mediated cyclisations 9 and the Maitland-Japp reaction. 10 Conversely, construction of the 2,6trans-THP ring is almost exclusively limited to either nucleophilic addition to cyclic hemiacetals via an oxocarbenium ion 11,12 or kinetically controlled oxy-Michael reactions, 3,4 though in the latter case the trans-selectivity is often only moderate.
We recently reported the synthesis of substituted dihydropyran-4-ones (DHPs), by extension of the Maitland-Japp reaction, 17 a method which is complementary to the Diels-Alder route popularised by Danishfesky. 18 We then converted these DHPs into 2,6-cis-THPs. 17 This strategy enabled us to complete syntheses of "Civet" and a fully functionalised model A-ring of lasonolide A. Given the dearth of methods for the construction of 2,6-trans-THP rings we turned our attention to the development of a new method for the selective synthesis of 2,6-trans-THPs. We envisaged that 2,6-trans-THPs could be formed from the conjugate addition of a carbon nucleophile to the double bond of Maitland-Japp DHPs such as 5. We rationalised that the stereoelectronic preference for axial addition of a nucleophile to the double bond would generate a 2,6-trans-THP with the opportunity to trap the resultant enolate, which would allow for further functionalisation of the THP-ring (Fig. 2).

Synthesis of dihydropyran-4-ones
In order to investigate the formation of 2,6-trans-THPs we had to prepare DHPs 5. To this end we employed the conditions we had used for the synthesis of C2-substituted DHPs (an orthoamide or orthoester in toluene), 17 however, when we used the dimethyl acetal of N,N-dimethylformamide and δ-hydroxyβ-ketoesters 7, complex mixtures of products resulted. Our initial results suggested that there was an inherent instability in the DHPs 5 that was not apparent in their C2-substituted counterparts, this was particularly noticeable during attempted isolation by chromatography on silica gel (2D TLC showed multiple interconverting spots). However, if the crude reaction mixture was exposed to a Gilman cuprate, it was possible to isolate some 2,6-trans THPwith the exception of 7a which gave a moderate isolated yield of DHP 5a. Following considerable investigation we realised that the Knoevenagellike condensation of the orthoamide occurred but the oxy-Michael cyclisation to give the DHP did not. This issue could be rectified by performing the reaction with only one equivalent of orthoamide in CH 2 Cl 2 , rather than PhMe, followed by the addition of BF 3 ·OEt 2 to promote cyclisation, resulting in a 92% crude mass balance of 5a which could be used crude, without the need for purification (Scheme 1).
These conditions proved general for the synthesis of a range of C2-unsubstituted, C6-substituted DHPs 5 (Table 1). In addition to the 2-furyl group 5a, other heteroaromatic substituents could be incorporated 5h, as well as phenyl 5b. n-Alkyl and branched alkyl substituents are readily tolerated 5c and 5d, along with alkene-containing side chains 5f and 5g. Perhaps the most encouraging, as it allows further elaboration of the C6-side chain, is the realisation that TIPS-protected alcohols can also be incorporated 5e.
With a range of DHPs to hand we were now in a position to study to formation of 2,6-trans-substituted THPs.
Conversion of dihydropyran-4-ones to 2,6-transtetrahydropyran-4-ones When DHPs 5 were treated with a range of Gilman cuprates, Ph 2 CuLi, Me 2 CuLi and Bu 2 CuLi in the presence of TMSCl at −78°C in THF, it was found that conjugate addition occurred smoothly to yield the 2,6-trans-THPs in a mixture of enol and keto-forms 8/9 ( Table 2). Addition of Ph 2 CuLi generated the 2,6-trans-THPs exclusively as the enol tautomer 8. However, use of Me 2 CuLi and Bu 2 CuLi generated mixtures of enol-keto tautomers 8 and 9 of the 2,6-trans-THPs. For the purposes of characterisation, these tautomers were converted into enol acetates 10 by the action of Ac 2 O, pyridine and DMAP.
We rationalise that 2,6-trans-THPs exist as a mixture of keto/enol tautomers because either the C2 or C6 substituent must be axial. The penalty for having an axial substituent may  be partly relieved by enolisation as this allows for the formation of an intramolecular H-bond and the reduction of a 1,3-diaxial interaction for the axial group. Therefore, in order to definitively characterise the 2,6-trans-THP products the keto/enol mixture was treated with Ac 2 O, pyridine and DMAP to form enol acetates 10, where the 2,6-trans-THP stereochemical configuration was confirmed by analysis of the 1 H NMR and NOE data (Fig. 3). In the representative case of 10h there was a strong NOE correlation between C2 methyl group and H6 of 2.3% and a NOE correlation between C2 methyl group and H5α of 1.86%.
Conversion of dihydropyran-4-ones to 3,6-disubstituted and 3,3,6-trisubstituted tetrahydropyran-4-ones With the development of a successful strategy for the synthesis of 2,6-trans-THPs we sought to extend the scope for the conver-sion of DHPs 5 into THPs with other substitution patterns. We considered the possibility that 3,6-disubstituted-THPs could be accessed by the conjugate reduction of the C2-C3 double bond. When DHPs 5 were treated with L-Selectride at −78°C and quenched, a range of 3,6-disubstituted THPs 11 were formed in good yields; the enol tautomer was the major product in all cases, with small amounts of the keto-tautomer present. In order to aid characterisation the product mixture was converted into the enol acetate 12 by the action of Ac 2 O, pyridine and DMAP (Table 3). In all cases studied we could not detect products from reduction of either the ketone or the ester carbonyl groups.
The addition of L-Selectride to DHPs 5 initially generated an enolate which was quenched upon workup to give 3,6-disubstituted THPs 11. We wondered if it would be possible to intercept the enolate with a carbon electrophile to form 3,3,6trisubstituted THPs. Alkyl halides methyl iodide, allyl bromide and benzyl bromide were investigated (Table 4).
We reasoned that delivery of hydride would occur from the pseudo-axial trajectory and the electrophilic quenching would occur from the opposite face of the THP ring. This should deliver THP products with a quaternary stereocenter at C3, in which the R and R 1 groups are cis to each other. No other diastereomer was detected in the 1 H NMR of the crude reaction mixture. The THP products 13 were characterised, and the relative stereochemical configuration confirmed, by 1 H NMR and NOE correlations. For example, in the representative case of 13i there was a clear NOE of 3.6% between H6 and H5α when H6 was irradiated. When H5α was irradiated a NOE to H6 of 2.26% was seen. There was a NOE of 3.16% between H5β and the benzyl CH 2 group, indicating that these were both axial (Fig. 4). The protocol gave the desired functionalisation with the halide electrophiles but, to our disappointment, we were unable to intercept the enolate with aldehyde electrophiles,   which probably reflects the inherent stability of the β-ketoester's enolate anion.

Synthesis of diospongin B
With procedures developed for the synthesis of highly substituted THP-rings, especially the less common and synthetically more challenging 2,6-trans-THP, we sought to demonstrate the utility of the approach by completing the total synthesis of the anti-osteoporotic 2,6-trans-THP-containing natural product diospongin B 2. Diospongin B is a diaryl heptanoid natural product which was isolated in 2003 from the rhizomes of Dioscorea spongiosa and was shown to exhibit potent inhibitory activity on bone resorption induced by parathyroid hormone. 16 The activity of diospongin B is comparable to calcitonin, a drug currently used to treat osteoporosis, and this has led to a number of total syntheses being reported for it and its 2,6-cisdiastereomer, diospongin A. 19 Our synthesis (Scheme 2) began with the Maitland-Japp formation of DHP 5g in 97% yield using the dimethylacetal of N,N-dimethyl formamide. Conjugate addition of Ph 2 CuLi to 5g yielded 2,6-trans-THP 8f in 91%. Microwave-mediated decar-boxylation in wet DMF generated the desired tetrahydropyran-4-one, 20 which was in turn reduced with L-Selectride to give THP 14 as the major diastereomer (9 : 1) with the correct relative stereochemical configuration for diospongin B. The stereochemical configuration of 14 was confirmed by H2 being coupled to both H3α and H3β with J = 4. 4 Hz indicating its equatorial position, H6 was coupled to H5β J = 9. 1 Hz and H5α J = 5. 0 Hz, indicating its axial position while H4 was coupled to H5β J = 9. 3 Hz, H5α J = 4.5 Hz, H3β J = 9.0 Hz and H3α J = 4.0 Hz indicating its axial orientation. Additionally, H2 only had NOE correlations to H3α of 1.33% and to H3β of 1.89%, H4 had NOE correlations to H6 of 1.23%, to H3α of 1.58% and to H5α of 2.59% (Fig. 5). The synthesis was completed by MOM-protection of the free hydroxyl in 60% yield, and Wacker oxidation of the double bond to give 15 in 70% yield. 19f The final step was the removal of the MOM protecting group, which was achieved by the action of aqueous HCl 19f and generated diospongin B 2 in 58% yield. Spectroscopic data for our sample of diospongin B 2 were identical to those reported in the literature. 19

Conclusions
We have developed a modification of the Maitland-Japp reaction using orthoamides which provides access to a range of 6-substituted-2H-dihydropyran-4-ones in good yields. These 2H-dihydropyran-4-ones can be converted into tetrahydropyran products with uncommon substitution patterns which are found in a number of biologically active natural products. 2,6trans-Tetrahydropyran-4-ones are obtained by the stereoselective addition of Gilman cuprates to 6-substituted-2H-dihydropyran-4-ones. Tetrahydropyrans with the 3,6-substitution pattern are accessed by the conjugate addition of L-Selectride, while 3,3,6-substitution pattern are obtained by trapping the enolate formed on addition of L-Selectride with a carbon electrophile. The utility of these procedures was demonstrated by their use in the total synthesis of diospongin B, a natural product with potent anti-osteoperotic activity. This work provides a new route to uncommon tetrahydropyran substitution patterns and may ease the synthesis of a significant number of natural products containing these units.

General methods
Thin layer chromatography was performed on aluminium plates coated with Merck silica gel 60 F 254 . The plates were developed using ultraviolet light, acidic aqueous ceric ammonium molybdate, basic aqueous potassium permanganate or ethanolic anisaldehyde. Flash column chromatography was performed with the solvent systems indicated in the appropriate experimental procedure. The stationary phase was silica gel 60 (220-240 mesh), unless stated otherwise. Dichloromethane was distilled from calcium hydride; THF and Et 2 O were distilled from sodium-benzophenone ketyl radical; toluene was dried over sodium wire; hexane was distilled prior to use. All other solvents and reagents were used as received from commercial suppliers. 1 H NMRs were recorded at ambient temperature at either 400 MHz or 500 MHZ and 13 C NMRs were recorded at ambient temperature at either 100 MHz or 125 MHz. Mass spectrometry was performed using ES ionisation.
General procedure for the synthesis of 6-substituted-2H-dihydropyran-4-ones 5 N,N-Dimethylformamide dimethyl acetal (0.03 mL, 0.20 mmol) was added to a stirred solution of δ-hydroxy-β-ketoester 7 (0.2 mmol) in dry dichloromethane (2 mL) at room temperature. After stirring at this temperature for 45 minutes, BF 3 ·OEt 2 (0.03 mL, 0.20 mmol) was added. The reaction was stirred at room temperature and monitored by TLC (hexane-ethyl acetate). Upon completion the mixture was diluted with EtOAc (40.0 mL) and washed with sat. aq. NaHCO 3 (10.0 mL). The aqueous layer was extracted with EtOAc (15.0 mL) and the combined organic extracts were washed with brine (10.0 mL), dried over MgSO 4 and concentrated in vacuo to give the crude DHP