[reaction: see text] Platinum(II) and an unusual cationic gold(I) complex were identified as mild catalysts for the room temperature cycloisomerization or tandem hydroalkoxylation/acetal formation of unactivated internal alkynols. Under the appropriate conditions, 5-endo, 5-exo, 6-endo, and 6-exo cycloisomerization modes are all available.

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E n a n t i o s e l e c t i v e S t r e c k e r R e a c t i o n P r o m o t e d b y C h i r a l N Abstract: The asymmetric Strecker reaction of aldimines and trimethylsilyl cyanide promoted by 1 equivalent of chiral N-oxide affords the corresponding a-amino nitriles with enantioselectivities of up to 73% most effectively for electron-deficient aldimines under mild reaction conditions. Two new resolving methods and one new synthetic methodology were developed to obtain three novel chiral N-oxides.The ever-increasing interest in non-proteinogenic a-amino acids in a variety of scientific disciplines has prompted the development of numerous methods for asymmetric synthesis of a-amino acids. The enantioselective addition of cyanide to imines (asymmetric Strecker reaction) constitutes one of the most direct and viable strategies for the asymmetric synthesis of a-amino acids. Recently, significant progress has been made in the development of enantioselective catalytic version.

Scheme 1. Structures of leucosceptroids A-D. Scheme 2. Retrosynthetic analysis of leucosceptroid B. R = protecting group.

We report the first total synthesis of bolivanine in a 14-step pathway involving the synthesis of onoseriolide. Our synthesis features a palladium-catalyzed intramolecular cyclopropanation involving an allylic metal carbene and a Diels-Alder/intramolecular hetero-Diels-Alder cascade, allowing the single-step assembly of a tricyclic system with proper configuration. The synthetic efforts validate our modified biogenetic hypothesis and allow us to confirm the absolute configuration of bolivianine.

In this article, we describe our efforts on the total synthesis of bolivianine (1) and isobolivianine (2), involving the synthesis of onoseriolide (3). The first generation synthesis of bolivianine was completed in 21 steps by following a chiral resolution strategy. Based on the potential biogenetic relationship between bolivianine (1), onoseriolide (3), and β-(E)-ocimene (8), the second generation synthesis of bolivianine was biomimetically achieved from commercially available (+)-verbenone in 14 steps. The improved total synthesis features an unprecedented palladium-catalyzed intramolecular cyclopropanation through an allylic metal carbene, for the construction of the ABC tricyclic system, and a Diels-Alder/intramolecular hetero-Diels-Alder (DA/IMHDA) cascade for installation of the EFG tricyclic skeleton with the correct stereochemistry. Transformation from bolivianine to isobolivianine was facilitated in the presence of acid. The biosynthetic mechanism and the excellent regio- and endo selectivities in the cascade are well supported by theoretical chemistry based on the DFT calculations.

A range of stable ytterbium­(II) amino ether–phenolato amido complexes of the type {LONxOy }­Yb­{N­(SiMe3)2}, together with a congeneric samarium­(II) and two calcium and strontium amido and alkyl derivatives, have been synthesized. They constitute very active, fully regioselective (anti-Markovnikov), and chemoselective precatalysts for the intermolecular hydrophosphination of styrene derivatives with PhPH2, achieving TONs up to 2150, TOFs up to 30 h–1 and chemoselectivities in the region of 95–99%. The ytterbium­(II) precatalysts, among which is most prominently {LONO4}­Yb­{N­(SiMe3)2} (3), where the ligand possesses a 15-c-5-aza-crown ether side arm, outperform their related calcium analogues, and the activity of the catalyst increases substantially with the denticity of the ligand. The rate law rate = k[p-tBu-styrene]­[3] was established following kinetic monitoring of the hydrophosphination of p-tBu-styrene and PhPH2 catalyzed by 3. The kinetic studies also revealed that the reactions are entropically driven and that reaction rates increase when electron-withdrawing groups are introduced at the para position of the styrenic substrate. It is proposed that these catalyzed hydrophosphination reactions proceed by rate-limiting olefin insertion into the [Yb]–phosphide bond. The chemoselective one-pot, two-step double hydrophosphination of PhPH2 with 2 equiv of styrene yields tertiary phosphines. It obeys an unusual kinetic profile where formation of the tertiary phosphine starts only when complete consumption of PhPH2 is first ensured. This sequence was used to obtain asymmetric tertiary phosphines with excellent selectivity.