Enantioselective Total Synthesis of (-)-Monic Acid C via Carbosulfenylation of Dihydropyran

Article abstract of DOI:10.1021/jo00260a020

A stereoselective synthesis of (-)-monic acid C (5) was accomplished in a linear sequence of 22 steps beginning from dihydropyran and with (-)-1-borneol as a chiral auxiliary. 2-(1-Bornyloxy)pyrans 10 and 11 were converted via trans epoxides 12 and 13 to hydroxy selenides 16 and 17.Selenide 16 was transformed to vinyl ether 20, which underwent Claisen rearrangement to 22.The latter, after oxidation to carboxylic acid 24, was treated with stannic chloride to give the γ-lactone 26.A parallel sequence from 17 brought this selenide, via 29 and its Claisen rearrangement product 31, into convergence with 26.Carbosulfenylation of 26 in the presence of 2-(trimethylsiloxy)propene was accompanied by elimination and led to 33 and its trans isomer 34.Cis hydroxylation of 33, protection of the resulting diol, and Horner-Emmons condensation of 41 with tert-butyl dimethyl-phosphonoacetate gave 43.Attachment of the second side chain of 5 was effected by means of a Julia olefination of 46 with sulfone 65, prepared from ethyl (2S,3S)-3-hydroxy-2-methylbutanoate.The resulting substituted tetrahydropyran 68, after removal of the three protecting groups, yielded 5.