An approach to cytochalasins: Diels-Alder addition of alpha,beta-unsaturated imides

Intramolecular cyclizations of polyketide biosynthesis: mining for a "Diels-Alderase"?

Despite the large number of naturally occurring metabolites existing for which enzymatic Diels-Alder reactions have been proposed as a key biosynthetic step, the actual number of enzymes thus far identified for these transformations is incredibly low. Even for those few enzymes identified, there is currently little biochemical or mechanistic evidence to support the label of a "Diels-Alderase." For several families of polyketide metabolites, the transformation in question introduces a rigid, cross-linked scaffold, leaving the remaining peripheral modifications and polyketide processing to provide the variation among the related metabolites. A detailed understanding of these modifications--how they are introduced and the tolerance of enzymes involved for alternate substrates--will strengthen biosynthetic engineering efforts toward related designer metabolites. This review addresses intramolecular cyclizations that appear to be consistent with enzymatic Diels-Alder transformations for which either the responsible enzyme has been identified or the respective biosynthetic gene cluster for the metabolite in question has been elucidated.

Synthesis of alpha,beta-unsaturated gamma-lactams via asymmetric iridium-catalysed allylic substitution

Syntheses of alpha,beta-unsaturated gamma-lactams are described that are based on ring-closing metathesis in combination with enantioselective Ir-catalysed allylic amination using N-Boc-N-(but-2-enoyl)-amine as a pronucleophile. As an example application, the synthesis of a Baclofen analogue is presented.

Studies in Heteroelement-Based Synthesis

An account of studies in our group over 30 years is presented, with highlights taken from events that began with our plans for medium ring synthesis, but were soon diverted to organophosphorus chemistry, and that eventually resulted in the investigation of sulfur ylide ring expansions, thiocarbonyl chemistry, azomethine ylides, organotin and organoboron methodology, and control of relative and absolute configuration.

A synthesis of C(16),C(18)-bis-epi-cytochalasin D via Reformatsky cyclization

Triene 5 has been prepared by the E-selective olefination of aldehyde 12 with the ylide 11. Several alternative syntheses of 12 were evaluated, and the successful route involved conversion of 22 into the vinyl ether 23 by Petasis olefination, followed by Claisen rearrangement. Diels-Alder cycloaddition of 5 with 4 gave the adduct 6 in 77% yield, and Reformatsky cyclization under dilution conditions afforded 10 (67%). After conversion to enol silane 32, oxidation with dimethyldioxirane produced 34. Conversion to a key intermediate 38 using electrophilic selenenylation and selenoxide rearrangement, followed by enolate alkylation and deprotection, gave 43. The X-ray crystal structure of 43 was determined to prove the stereochemistry.