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 C₂-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 1a were achieved by treatment of the bis-BOC carbonate 13 with IBr in toluene at −80 °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.