Spiroacetal subunits

The CD-spiroacetal subunit of the spongistatins proved to be of an appropriate level of complexity that several different synthetic strategies were evaluated. Access to the desired spiroacetal 5 was readily achieved by acid catalysed equilibration of the mixture of spiroacetals in aprotic solvents, followed by separation and recycling of the undesired isomer. Furthermore, the 1,5-anti aldol reaction of methyl ketones proved invaluable for construction of certain portions of the target molecule. 

The chiral lactone (165) used earlier in a synthesis of the spiroacetal subunit of milbemycin Bg, has now been applied in the... 

Paterson, I., Oballa, R.M., and Norcross, R.D. (1996) Studies in marine macrolide synthesis Stereocontrolled synthesis of the AB-spiroacetal subunit of spongistatin 1 (altohyrtin A). Tetrahedron Lett., 37, 8581-8584. 

Paterson, 1. and Coster, M.J. (2002) Total synthesis of altohyrtin A (spongistatin 1) an alternative synthesis of the CD- spiroacetal subunit C16-C28. Tetrahedron Lett., 43,3285-3289. 

The similar reaction of 2-alkyl-l,3-disilyloxypropanes with chiral ketones is highly selective and has been used to prepare spiroacetal starting materials for an asymmetric synthesis of a-tocopherol subunits (eq 12). ... 

---