A convenient method for 3-pyrroline synthesis

C-H Insertion by Alkylidene Carbenes To Form 1,2,3-Triazines and Anionic [3 + 2] Dipolar Cycloadditions To Form Tetrazoles: Crucial Roles of Stereoelectronic and Steric Effects

The synthesis of 1,2,3-triazines and bicyclic tetrazoles from α-azido ketones is described. The common intermediate generated from lithiated trimethylsilyldiazomethane and α-azido ketones diverges depending on the steric bulk of the substituents. The formation of 1,2,3-triazines via a C-H insertion of alkylidene carbene to form 3-azidocyclopropene, followed by its rearrangement, is supported by density functional theory calculations. Tetrazole formation proceeds via a facile anionic [3 + 2] dipolar cycloaddition between a lithiated diazo moiety and an azido group facilitated by the chelation of a lithium ion.

Dehydrative fragmentation of 5-hydroxyalkyl-1H-tetrazoles: a mild route to alkylidenecarbenes

The development of a mild, base-free method for the generation of alkylidenecarbenes is reported. Treatment of 5-hydroxyalkyl-1H-tetrazoles with carbodiimides generates products arising from the 1,2-rearrangement or [1,5]-C-H bond insertion of a putative alkylidenecarbene. Formation of this divalent intermediate is proposed to occur by way of a tetraazafulvene, which undergoes extrusion of 2 mol of dinitrogen. Details of this methodology, its application to the synthesis of combretastatin A-4, and an improved route to 5-hydroxyalkyl-1H-tetrazoles are described.

Copper-catalyzed multicomponent reaction: facile access to functionalized 5-arylidene-2-imino-3-pyrrolines

The synthesis of a novel class of 2-imino-5-arylidene-3-pyrrolines via a copper-catalyzed multicomponent reaction of sulfonyl azides with alkynes and aziridines is described. The protocol is efficient and general.

Reaction of alpha-(N-carbamoyl)alkylcuprates with enantioenriched propargyl electrophiles: synthesis of enantioenriched 3-pyrrolines

Enantioenriched propargyl mesylates or perfluorobenzoates react with alpha-(N-carbamoyl)alkylcuprates to afford scalemic alpha-(N-carbamoyl) allenes which undergo N-Boc deprotection and AgNO3-promoted cyclization to afford N-alkyl-3-pyrrolines. The synthetic sequence proceeds under optimal conditions with no loss of enantiopurity relative to the starting propargyl alcohols prepared by asymmetric addition of terminal alkynes to aldehydes.

An Efficient Enantioselective Synthesis of (2R)-Hydroxymethyl Glutamic Acid and an Approach to the (2R)-Hydroxymethyl-Substituted Sphingofungins

We have developed a short enantioselective synthesis of (2R)-hydroxymethyl glutamic acid (HMG) starting from Garner's aldehyde using an alkylidene carbene 1,5-CH insertion as a method to construct the quaternary stereocenter. A variety of conditions were examined for the oxidative cleavage of the key cyclopentene intermediate and we found that RuCl3/NaIO4 led directly to the desired amino bis-acid product. We were also able to show that oxidative cleavage of the cyclopentene 1,5-CH insertion product could be used to produce the amino acid-containing skeleton of the sphingofungin family of natural products.

Enantioselective total syntheses of (−)-clasto-lactacystin β-lactone and 7-epi-(−)-clasto-lactacystin β-lactone

An alkylidene carbene 1,5-CH insertion has been used as a key step in an efficient enantioselective total synthesis of (-)-clasto-lactacystin beta-lactone, and its C7-epimer. An additional noteworthy feature of the synthesis is the use of a novel oxidative deprotection procedure, utilizing DMDO, for the conversion of a late-stage benzylidene acetal into a primary alcohol and a secondary benzoate ester.

A formal synthesis of (+)-lactacystin

A synthetic route to the neurotrophic agent (+)-lactacystin has been developed utilizing a base-promoted intramolecular alkylidenecarbene C-H insertion as the key transformation.