A novel sulfonamide non-classical carbenoid: a mechanistic study for the synthesis of enediynes.

Alkynyl sulfonamides undergo sequential 1,4- then 1,2-addition/rearrangement with lithium acetylides to yield enediynes in the absence of any promoters or catalysts. Mechanistic investigations suggest that the reaction proceeds via 1,4-conjugate addition of the nucleophile to the unsaturated system to give a key alkenyl lithium species which is stabilised by an intramolecular coordination effect by a sulfonamide oxygen atom. This species can be considered a vinylidene carbenoid given the carbon atom bears both an anion (as a vinyllithium) and a leaving group (the sulfonamide).

Mechanistic interrogation of the asymmetric lithiation-trapping of N-thiopivaloyl azetidine and pyrrolidine.

A fundamental mechanistic study of the s-BuLi/chiral diamine-mediated lithiation-trapping of N-thiopivaloyl azetidine and pyrrolidine is reported. We show that lithiated thiopivalamides are configurationally unstable at -78 °C. Reaction then proceeds via a dynamic resolution of diastereomeric lithiated intermediates and this accounts for the variable sense and degree of asymmetric induction observed compared to N-Boc heterocycles.

On the synthesis of α-amino sulfoxides.

A synthetic study on the preparation of N-Boc α-amino sulfoxides has revealed an unexpected instability which is believed to be due to α-elimination of the sulfoxide to give an iminium ion. Full synthetic details are reported on two main synthetic routes: lithiation and sulfinate trapping of N-Boc heterocycles and oxidation of N-Boc α-amino sulfides. Six novel α-amino sulfoxides were successfully prepared and isolated.

Preparation and reactions of enantiomerically pure α-functionalized Grignard reagents.

A strategy for the generation of enantiomerically pure α-functionalized chiral Grignard reagents is presented. The approach involves the synthesis of α-alkoxy and α-amino sulfoxides in ≥99:1 dr and ≥99:1 er via asymmetric deprotonation (s-BuLi/chiral diamine) and trapping with Andersen's sulfinate (menthol derived). Subsequent sulfoxide → Mg exchange (room temperature, 1 min) and electrophilic trapping delivers a range of enantiomerically pure α-alkoxy and α-amino substituted products.

Total synthesis of potent antifungal marine bisoxazole natural products bengazoles A and B.

The bengazoles are a family of marine natural products that display potent antifungal activity and a unique structure, containing two oxazole rings flanking a single carbon atom. Total syntheses of bengazole A and B are described, which contain a sensitive stereogenic centre at this position between the two oxazoles. Additionally, the synthesis of 10-epi-bengazole A is reported.

Highly stereoselective Michael addition reactions of CamTHP*-desymmetrized glycinamide for the synthesis of functionally dense amino acid derivatives.

The camphor-derived tetrahydropyran (camTHP*)-desymmetrized glycinamide 1 undergoes efficient and highly diastereoselective lithium enolate Michael additions to nitro olefins, alpha,beta-unsaturated ketones, esters, and lactones. Straightforward manipulation of these products affords 3-substituted pyroglutamides and beta-aryl-alpha,gamma-diamino acid derivatives, highlighting the ease of synthesis of enantiomerically enriched, functionally dense molecules using this novel building block. [reaction: see text]

CamTHP*OH: a camphor-derived delta-lactol auxiliary for the effective desymmetrization of attached glycinamide residues. Asymmetric synthesis of alpha-amino carbonyl compounds.

Stereoselective allylation of camphor and subsequent terminal hydroformylation affords a new delta-lactol auxiliary (camTHP*OH) on multigram scale. Stereoselective condensation with glycine dimethylamide and Cbz protection affords a camTHP*-desymmetrized glycinamide building block which undergoes efficient and highly diastereoselective metal enolate alkylation reactions. Acid-mediated deprotection affords the N-Cbz-protected alpha-amino amide products which may be converted directly to alpha-amino ketones on treatment with Grignard or organolithium reagents without loss of stereochemical integrity.