Bidirectional asymmetric allylboration.: A convenient asymmetric synthesis of C2-symmetric 3-methylenepentane-1,5-diols and rapid access to C2-symmetric spiroketals

The double allylboration of aldehydes using 1,3-bis(diisopinocampheylboryl)-2-methylenepropanes(R,R)-3 and (S,S)-3 under Brown's salt-free conditions provides Ct-symmetric 3-methylenepentane-1,5-diols 1 in excellent enantiomeric excess. The absolute stereochemistry of the products was confirmed by a single-crystal X-ray study of bis-Mosher ester 6g. Desymmetrization and further functionalization of diol la were achieved by treatment of the bis-BOC carbonate 13 with IBr in toluene at -80 degrees C to give cyclic iodocarbonate 14 as a single diastereomer. This methodology is also applicable in natural product synthesis; enantiomerically pure spiroketals 1,7-dioxaspiro[5.5]-undecanes 18 and 25, the latter representing an expedient synthesis of the AB ring system of the spongistatins 20, were easily accessed from simple starting materials in excellent yields and selectivities.