title: A practical and efficient synthesis of the C-16-C-28 spiroketal fragment (CD) of the spongistatins.
abstract: A practical and efficient route to the CD spiroketal (C-16-C-28) of the spongistatins is reported. Two stereocenters are introduced from chiral building blocks with the remainder introduced by substrate-controlled transformations. The key beta-keto-1,3-dithiane intermediate is generated by a dithiol conjugate addition to an ynone and the 1,3-dithiane unit in the C-ring plays a key role in the spiroketalization and subsequent epimerization. The synthesis requires 24 steps, with a longest linear sequence of 19 steps in an overall yield of 14.5% (for the longest linear sequence). [reaction: see text]
authors.affiliation: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Gaunt
authors.forename: David F
authors.initials: DF
authors.lastname: Hook
authors.forename: Huw R
authors.initials: HR
authors.lastname: Tanner
authors.forename: Steven V
authors.initials: SV
authors.lastname: Ley