Asymmetric total synthesis of (−)-melotenine A

Chemoselective Intramolecular Morita-Baylis-Hillman Reaction; Acrylamide and Ketone as Sluggish Reacting Partners on a Labile Framework

Chemoselectivity is an important issue frequently encountered while working over labile precursors. Carbonyl compounds with a heteroatom at the β carbon are sensitive precursors because they are prone to elimination under different conditions. Morita-Baylis-Hillman (MBH) reaction, although a widespread method for C-C bond formation, has its own limitations. Acrylamide and ketone are such limitations of the MBH reaction. Using them together for an intramolecular MBH (IMBH) reaction on a labile framework prone to elimination is a significant 2-fold synthetic challenge. A highly chemoselective IMBH reaction on such precursors has been established using 1,4-diazabicyclo[2.2.2]octane (DABCO) as a promoter. The protocol leads to quick access to a diversely substituted and functionalized piperidone framework in high yields. Various substitution patterns in the form of 34 successful examples have been studied. A diastereoselective version and tolerance to various functional and protecting groups are the added advantages of the developed methodology. A tertiary carbon at the β position of ketone, however, led to complete reversal of selectivity and gave only the elimination product. Control experiments toward a better understanding of the substitution pattern, role of catalyst, and mechanistic study have been carried out. As an application of the IMBH adduct, a one-step allylic rearrangement for the dihydropyridone framework has also been demonstrated.

Application of The Dess-Martin Oxidation in Total Synthesis of Natural Products

Dess-Martin periodinane (DMP), a commercially available chemical, is frequently utilized as a mild oxidative agent for the selective oxidation of primary and secondary alcohols to their corresponding aldehydes and ketones, respectively. DMP shows several merits over other common oxidative agents such as chromiumand DMSO-based oxidants; thus, it is habitually employed in the total synthesis of natural products. In this review, we try to underscore the applications of DMP as an effective oxidant in an appropriate step (steps) in the multi-step total synthesis of natural products.

Synthesis of (-)-Melodinine K: A Case Study of Efficiency in Natural Product Synthesis

Efficiency is a key organizing principle in modern natural product synthesis. Practical criteria include time, cost, and effort expended to synthesize the target, which tracks with step-count and scale. The execution of a natural product synthesis, that is, the sum and identity of each reaction employed therein, falls along a continuum of chemical (abiotic) synthesis on one extreme, followed by the hybrid chemoenzymatic approach, and ultimately biological (biosynthesis) on the other, acknowledging the first synthesis belongs to Nature. Starting materials also span a continuum of structural complexity approaching the target with constituent elements on one extreme, followed by petroleum-derived and "chiral pool" building blocks, and complex natural products (i.e., semisynthesis) on the other. Herein, we detail our approach toward realizing the first synthesis of (-)-melodinine K, a complex bis-indole alkaloid. The total syntheses of monomers (-)-tabersonine and (-)-16-methoxytabersonine employing our domino Michael/Mannich annulation is described. Isolation of (-)-tabersonine from Voacanga africana and strategic biotransformation with tabersonine 16-hydroxylase for site-specific C-H oxidation enabled a scalable route. The Polonovski-Potier reaction was employed in biomimetic fragment coupling. Subsequent manipulations delivered the target. We conclude with a discussion of efficiency in natural products synthesis and how chemical and biological technologies define the synthetic frontier.

Total Synthesis of (+)-epi-Condyfoline

Herein, we report the first asymmetric total synthesis of aspidospermatan indole alkaloid (+)-epi-condyfoline (1) in 15 steps from commercially available 2-methylindole-3-carboxaldehyde. Key steps include (1) our domino Michael/Mannich annulation method of N-sulfinyl metallodienamines to set three contiguous stereocenters, (2) LiHMDS-mediated cyclization of an ω-tosyloxy N-sulfinamide to prepare the signature indole-fused 2-azabicyclo[3.3.1]nonane framework, and (3) DMTSF-promoted spirocyclization of a dithioacetal intermediate to access the final pyrrolidine ring. Functional group manipulations delivered the targeted alkaloid (+)-epi-condyfoline (1) in 13 steps and 1.25% overall yield from N-sulfinylimine (+)-8.

An efficient approach to trans-4-hydroxy-5-substituted 2-pyrrolidinones through a stereoselective tandem Barbier process: divergent syntheses of (3R,4S)-statines, (+)-preussin and (−)-hapalosin

Natural products of (+)-preussin and (−)-hapalosin have been asymmetrically synthesized through the stereoselective tandem Barbier process.

An enantioselective organocatalytic intramolecular Morita–Baylis–Hillman (IMBH) reaction of dienones, and elaboration of the IMBH adducts to fluorenones

An efficient synthesis of cyclopenta-fused arenes and heteroarenes has been achieved via an enantioselective organocatalytic intramolecular Morita–Baylis–Hillman (IMBH) reaction of dienones. The IMBH-adducts were transformed to fluorenones in a serendipitous manner.