Oxazolidinones and 2,5-Dihydrofurans via Zinc-Catalyzed Regioselective Allenylation Reactions of l-α-Amino Aldehydes

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Stereoselective Synthesis of (4S,5S)-5-Vinyloxazolidin-2-one-4-carboxylate as a β-Vinylserine Synthetic Equivalent by Vinyl Grignard Addition to an N-Tosyl Version of Garner's Aldehyde

A highly efficient synthesis of a β-vinylserine synthetic equivalent is reported that exploits the stereodirecting effect of the N-toluenesulfonamide in an anti-diastereoselective (8.5:1) vinyl Grignard addition to an analogue of Garner's aldehyde. Both aryl and alkyl Grignards are shown to give increased anti-selectivity compared with N-Boc Garner's aldehyde.

Chemodivergent reactions

An important strategy for the efficient generation of diversity in molecular structures is the utilization of common starting materials in chemodivergent transformations. The most studied solutions for switching the chemoselectivity rely on the catalyst, ligand, additive, solvent, temperature, time, pressure, pH and even small modifications in the substrate. In this review article several processes have been selected such as inter- and intramolecular cyclizations, including carba-, oxa-, thia- and oxazacyclizations promoted mainly by Brønsted or Lewis acids, transition metals and organocatalysts, as well as radical reactions. Catalyst-controlled intra- and intermolecular cyclizations are mainly described to give five- and six-membered rings. Cycloaddition reactions involving (2+2), (3+2), (3+3), (4+1), (4+2), (5+2), (6+2) and (7+2) processes are useful reactions for the synthesis of cyclic systems using organocatalysts, metal catalysts and Lewis acid-controlled processes. Addition reactions mainly of carba- and heteronucleophiles to unsaturated conjugated substrates can give different adducts via metal catalyst-, Lewis acid- and solvent-dependent processes. Carbonylation reactions of amines and phenols are carried out via ligand-controlled transition metal-catalyzed multicomponent processes. Ring-opening reactions starting mainly from cyclopropanols, cyclopropenols and epoxides or aziridines are applied to the synthesis of acyclic versus cyclic products under catalyst-control mainly by Lewis acids. Chemodivergent reduction reactions are performed using dissolving metals, sodium borohydride or hydrogen transfer conditions under solvent control. Oxidation reactions include molecular oxygen under solvent control or using different dioxiranes, as well as chemodivergent palladium catalyzed cross-coupling reactions using boronic acids are applied to aromatic and allenic compounds. Other chemodivergent reactions such as alkylations and allylations under transition metal catalysis, dimerization of acetylenes, bromination of benzylic substrates, and A³-couplings are performed via catalyst- or reaction condition-dependent processes.

Divergent Pd-catalyzed cross-coupling of allenyloxazolidinones to give chiral 1,3-dienes and vinyloxazolidinones

The divergent reactivity of 5-allenyloxazolidinones has been explored. This novel building block undergoes Pd(0)-catalyzed cross-coupling with boronic acids to form a wide range of chiral 1,3-dienes and pharmaceutically useful vinyloxazolidinones, the chemoselectivity being tightly controlled by a simple switch in additive.

An efficient Nozaki-Hiyama allenylation promoted by the acid derived MIL-101 MOF

A concise synthesis of the sulfonic acid-containing MIL-101 MOF catalyst was reported using commercially available materials. A series of characterization of as-synthesized MIL-101-SO3H including SEM, XRD, FTIR, BET and TGA was also demonstrated. Using MIL-101-SO3H as a catalyst, an efficient Nozaki-Hiyama allenylation reaction was achieved to generate various polyfunctionalized α-allenic alcohols in high yield and good selectivity. Taking advantage of the high acidity of the MIL-101-SO3H MOF structure, such transformations were also achieved under mild reaction conditions and short reaction times. Based on our observed evidence during this study, a mechanism was proposed involving a substrate activation/γ-nucleophilic addition reaction sequence. In addition, the MIL-101-SO3H catalyst can be recycled ten times during the Nozaki-Hiyama allenylation reaction without compromising the yield and selectivity.

γ-, Diastereo-, and Enantioselective Addition of MEMO-Substituted Allylboron Compounds to Aldimines Catalyzed by Organoboron-Ammonium Complexes

The first catalytic, broadly applicable, efficient, γ-, diastereo-, and enantioselective method for addition of O-substituted allyl-B(pin) compounds to phosphinoylimines (MEM=methoxyethoxymethyl, pin=pinacolato) is presented. The identity of the most effective catalyst and the optimal protecting group for the organoboron reagent were determined by consideration of the steric and electronic requirements at different stages of the catalytic cycle, namely, the generation of the chiral allylboronate, the subsequent 1,3-borotropic shift, and the addition step. Aryl-, heteroaryl-, alkenyl- and alkyl-substituted vicinal phosphinoylamido MEM-ethers were thus accessed in 57-92 % yield, 89:11 to >98:2 γ:α selectivity, 76:24-97:3 diastereomeric ratio, and 90:10-99:1 enantiomeric ratio. The method is scalable, and the phosphinoyl and MEM groups may be removed selectively or simultaneously. Utility is highlighted by enantioselective synthesis of an NK-1 receptor antagonist.

Total Synthesis of Natural Hyacinthacine C5 and Six Related Hyacinthacine C5 Epimers

The total synthesis of natural (+)-hyacinthacine C₅ was achieved, which allowed correction of its initially proposed structure, as well as six additional hyacinthacine C-type compounds. These compounds were readily accessible from two epimeric anti-1,2-amino alcohols. Keeping a common A-ring configuration, chemical manipulation occurred selectively on the B-ring of the hyacinthacine C-type products through methods of syn-dihydroxylation, S_N2 ring-opening of a cyclic sulfate, and also employing either ( R)- or ( R, S)-α-methylallyl amine for the Petasis borono Mannich reaction. Our small analogue library was then assessed for its glycosidase inhibitory potency against a panel of glycosidases. (-)-6- Epi-hyacinthacine C₅ and (+)-7- epi-hyacinthacine C₅ (compound names are based on the corrected structure of hyacinthacine C₅) proved most active, with inhibitory activities ranging between weak (IC₅₀ = 130 μM) and moderate (IC₅₀ = 9.9 μM) against the α-glucosidases of rat intestinal maltase, isomaltase, and sucrase, thus identifying potential new leads for future antidiabetic drug development.

Memory of Chirality Concept in Asymmetric Intermolecular Michael Addition of α-Amino Ester Enolates to Enones and Nitroalkenes

A highly stereoselective asymmetric intermolecular conjugate addition of α-amino ester derivatives to cyclic enones via the memory of chirality (MOC) concept in high yields with excellent diastereo- and enantioselectivity (dr >99:1, up to 99% ee) is reported. The applicability and the generality of the strategy was demonstrated by its further exploration to acyclic α,β-unsaturated ketone and aromatic nitroalkenes, resulting in the formation of δ-keto-α-amino ester derivative and γ-nitro-α-amino ester derivatives, respectively, with excellent ee and dr.