Tetrahydropyranyl ethers Subject

Tetrahydropyranyl (THP) ethers, another species known to be unstable to acid, have similarly been reported to be cleaved by solutions of iodine in methanol.209 At room temperature, cleavage of the THP ethers was complete in 1.5 to 8 h. As with the previous example using iodine in methanol at lower than reflux temperature, TBDMS ethers were stable to these conditions. The ability to tune the reactivity of the iodine in methanol system by simply controlling the temperature is of value in selective deprotection. This is even more useful when fluorine, known to remove only silyl ethers,105 is exploited. Given that methoxymethyl ethers, essentially acetals, are known to be cleaved under acidic conditions, it seems likely they too should be subject to removal by solutions of iodine in methanol. Sundry examples of deprotections using iodine in methanol are presented in Table IV. 

The starting material for the present synthesis was Wieland-Miescher ketone (24), which was converted to the known alcohol (25) by the published procedure [10], Tetrahydropyranylation of alcohol (25) followed by hydroboration-oxidation afforded the alcohol (26), which on oxidation produced ketone (27). Reduction of (27) with metal hydride gave the alcohol (28) (56%). This in cyclohexane solution on irradiation with lead tetraacetate and iodine produced the cyclic ether that was oxidized to obtain the keto-ether (29). Subjection of the keto-ether (29) to three sequential reactions (formylation, Michael addition with methyl vinyl ketone and intramolecular aldol condensation) provided tricyclic ether (30) whose NMR spectrum showed it to be a mixture of C-10 epimers. The completion of the synthesis of pisiferic acid (1) did not require the separation of epimers and thus the tricyclic ether (30) was used for the next step. The conversion of (30) to tricyclic phenol (31) was... 

Enantioselective synthesis ofa-hydroxy acid derivatives. Recently, Aggarwal has reported an enantioselective approach to the synthesis of a-hydroxy acid derivatives using trans- 1,3-dithiane- 1,3-dioxide. For example, reaction of trans-1,3-dithiane-1,3-dioxide with an aromatic aldehyde liberates the alcohol which is protected as the tetrahydropyranyl (THP) ether the resulting product may then be subjected to a Pummerer reaction, using trifluoroacetic anhydride, to give a thiolester. Transthiolesterification of this product using LiSEt gives the... 

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