Antonio Sánchez-Ruiz

Chiral sulfur ylides for the synthesis of bengamide E and analogues.

A new synthetic methodology of asymmetric epoxidation developed in our laboratories has been employed for the stereoselective synthesis of bengamide E (16) and analogues at the terminal olefinic position. In the event, the chiral sulfonium salt 30 was transformed into its corresponding sulfur ylide and reacted with aldehydes 21 and 44 to efficiently provide epoxy amides 31 and 45, respectively. To access the bengamides from these epoxy amides, we combined a synthetic strategy previously reported by us, using an olefin cross metathesis reaction to introduce various alkyl substituents at the terminal olefinic position of amide 33, with reactions mediated by palladium (Negishi or Suzuki couplings) from amide 49. This latter route of introduction of alkyl groups proved to be more efficient than the metathesis approach and allowed access to the generation of a wide array of new bengamide analogues.

A Highly Stereoselective Synthesis of Glycidic Amides Based on a New Class of Chiral Sulfonium Salts: Applications in Asymmetric Synthesis

A new type of chiral sulfonium salts that are characterized by a bicyclic system has been designed and synthesized from α-amino acids. Their corresponding ylides, which were prepared by basic treatment of the sulfonium salts, reacted smoothly with a broad array of simple and chiral aldehydes to provide trans-epoxy amides in reasonable to very good yields and excellent stereoselectivities (>98%). The obtained epoxy amides were found to be useful as synthetic building blocks. Thus, they were reduced into their corresponding epoxy alcohols and subjected to oxirane-ring-opening reactions with different types of nucleophiles.

Epi-, Epoxy-, and C2-Modified Bengamides: Synthesis and Biological Evaluation

With the objective of investigating the influence of structural modifications of the polyketide chain of the bengamides upon their antitumoral activities, we targeted the preparation of bengamide E analogues with modification of the stereochemistry at C-2 and at C-3, the substituent at the C-2 position, and the presence of oxirane rings. For the synthesis of these analogues, a new synthetic method for asymmetric epoxidation, developed in our laboratories, was employed utilizing the chiral sulfonium salts 22 and 23. In order to access 2-epi-bengamide E from these epoxy amides, a synthetic methodology, developed by Miyashita, allowed an oxirane-ring-opening reaction with a double inversion of the configuration. Alternatively, an aldol reaction provided access to the same analogue in a shorter and more efficient manner. Finally, biological evaluation of all of these bengamide E analogues demonstrated that the polyketide chain is essential for the antitumor activity of these natural products, not being amenable to structural or configurational modifications.

Epoxyamide-based strategy for the synthesis of polypropionate-type frameworks.

A new approach to the stereoselective synthesis of polypropionate-type frameworks is reported utilizing reactions of amide-stabilized sulfur ylides with chiral aldehydes. To establish a new strategy for macrolide fragment synthesis, the stereoselectivity of these reactions in the construction of epoxy amides was the most important aspect of this study. In this aspect, we found a strong influence of the protecting groups employed in the starting aldehydes upon the stereochemical outcome of their reactions with the sulfur ylide 1. Thus, numerous aldehydes showed remarkable stereofacial differentiation, providing a major diastereoisomer, in contrast to others that displayed a poor or no stereoselectivity. Despite the difficulties encountered for some cases with respect to their diastereomeric yields, we were able to prepare various stereotetrads and stereopentads, thus enhancing the synthetic value of this new methodology for the preparation of typical polypropionate frameworks found in many natural products, in particular the macrolide class of antibiotics.

The Chiron Approach to Pironetins: Synthesis of the δ‐Lactonic Fragment and Modified Building Blocks from D‐Glucal

Abstract The synthesis of the δ‐lactonic fragment of pironetin (1), a natural product with outstanding antimitotic properties, is reported. The synthesis was achieved from commercially available tri‐O‐acetyl‐D‐glucal (6) that was employed as starting material for the preparation of ethyl ketone 4, through a synthetic sequence that included a Ferrier rearrangement of 6 and suitable functional group manipulations of the resulting O‐glycoside 7 to obtain the 4‐ethyl glycoside 11, together with a series of 4‐C‐alkyl modified derivatives. The completion of the synthesis of 4 was performed via chain elongation at C‐6 by the introduction of a nitrile group and subsequent reduction, nucleophilic attack with ethyl magnesium bromide, and, finally, oxidation of the resulting alcohol 20.