Synthesis of the C(1)-C(16) fragment of bryostatins using an ‘ene’ reaction between an allylsilane and an alkynone

The zinc(II) iodide mediated ‘ene’ reaction between (4R)-4,5-bis-(tert-butyldimethylsilyloxy)-2-(trimethylsilylmethyl)pent-1-ene (43) and (5S,7R,9S)-5,11-dibenzyloxy-4,4-dimethyl-7,9-dihydroxy-7,9-O-isopropylideneundec-1-yn-3-one (53) gave the (E)-vinylsilane 54 with excellent stereoselectivity. Simultaneous deprotection and cyclisation via a stereoselective oxy-Michael reaction gave the bicyclic acetal 57 after treatment with trimethyl orthoformate. A synthesis of the ester 60 corresponding to the C(1)-C(16) fragment of the bryostatins was then completed by O-silylation, oxidative cleavage of the methylene group and a stereoselective condensation of the resulting ketone 59 with the chiral phosphonate 61.