ABSTRACT
Two approaches to the synthesis of compounds corresponding to the C17-C27 fragment of the 20-deoxybryostatins are described. The first approach is based on the palladium(0) catalysed coupling of tin enolates, generated in situ from enol acetates using tributyltin methoxide, with vinylic bromides. The vinylic bromides were prepared using the Sharpless asymmetric dihydroxylation to introduce the hydroxyl groups corresponding to those at C25 and C26 in the bryostatins. Following several steps to introduce alkynyl ester functionality, the stereoselective addition of a tributyltin cuprate followed by tributyltin-bromine exchange gave the required vinylic bromides. The palladium(0) catalysed couplings worked very well for enol esters containing thioether substituents and gave products with retention of the position and geometry of the trisubstituted double bond derived from the vinylic bromide. These were taken through to compounds corresponding to fully developed C17-C27 fragments ready for assembly of the 16,17-double-bond of bryostatins by Julia reactions. This chemistry was also applied to prepare intermediates suitable for incorporation into bryostatins by ring-closing metathesis but, in this case, the coupling reaction gave mixtures of products including both the required ßγ-unsaturated ketone and a conjugated diene formed by a competing Heck reaction. To avoid this problem, a second approach to compounds suitable for incorporation into a metathesis-based assembly of 20-deoxybryostatins was developed. In this organotin-free synthesis, the key step was the conjugate addition of an organic cuprate generated from allylmagnesium bromide to an alkynoate that gave the required (Z)-trisubstituted alkene with excellent stereoselectivity. This was converted into metathesis precursors in a few steps.
ABSTRACT
Preliminary studies into the use of ring-closing metathesis (RCM) in a convergent approach for the total synthesis of bryostatins are described. An ester that would have provided an advanced intermediate for a synthesis of a 20-deoxybryostatin by a RCM was prepared from an unsaturated acid and alcohol corresponding to the C1-C16 and C17-C27 fragments. However, studies of the formation of the C16-C17 double-bond by RCM were not successful and complex mixtures of products were obtained. To provide an insight into factors that may be involved in hindering RCM in this system, a slightly simplified C1-C16 acid and modified C17-C25 alcohols were prepared and their use for the synthesis of analogues of bryostatins was investigated. Although only low yields were obtained, it appeared that macrolides analogous to the bryostatins can be prepared by RCM, using the Grubbs II catalyst, if the precursors lack the two methyl groups at C18. RCM was not observed, however, for substrates in which these methyl groups were present.
Subject(s)
Bryostatins/chemical synthesis , Bryostatins/chemistry , Molecular StructureABSTRACT
Aryl(alkynyl)iodonium salts have been demonstrated to be valuable precursors to a diverse range of heteroaromatic ring systems including aryl[1,2-a]imidazopyridines. Successful application, using the recently described aryl(alkynyl)iodonium trifluoroacetate salts, is described, highlighting for the first time that the regioselectivity of this process is both counter-ion and concentration dependent. Studies with a carbon-13 labelled substrate established that the reactions of alkynyliodonium salts are highly complex and that multiple mechanistic processes appear to be underway simultaneously.
