A new method for
the synthesis of 2-aminoimidazole products is
described. The heterocyclic products are generated in good yields
via Pd-catalyzed carboamination reactions of N-propargyl
guanidines and aryl triflates. This methodology generates both a C–N
and C–C bond during the annulation step and facilitates the
rapid construction of 2-aminoimidazole products with different aryl
groups. The utility of this methodology was demonstrated in the total
synthesis of preclathridine A, preclathridine B, and dorimidazole
B from a single intermediate.
An organocatalytic three-component reductive coupling reaction between dimethyl phosphite, benzylidene pyruvates, and aldehydes is reported. A chiral triaryliminophosphorane catalyst promotes Pudovik addition, which is followed by phospha-Brook rearrangement to transiently generate enolates that are trapped stereoselectively by aldehydes. This reductive coupling provides vicinal polyfunctionalized stereocenters from readily available prochiral starting materials with excellent diastereoselectivity, enantioselectivity, and yield.
A new approach to the synthesis of substituted 5-membered cyclic guanidines is described. Palladium-catalyzed alkene carboamination reactions between acyclic N-allyl guanidines and aryl or alkenyl halides provide these products in good yield. This method allows access to a number of different cyclic guanidine derivatives in only two steps from readily available allylic amines.
Progress
toward a convergent approach for the enantioselective
synthesis of the Veratrum alkaloid jervine is presented.
The two requisite fragments were stereoselectively and efficiently
fashioned from economical and readily available reagents. Key reactions
include (a) a highly diastereoselective Ireland−Claisen rearrangement
to establish the necessary cis-relationship between
the amine and methyl group on the tetrahydrofuran E-ring; (b) a diastereoselective
selenoetherification reaction that enabled the assembly of the D/E
oxaspiro[4.5]decene in the needed configuration; and (c) an enzymatic
desymmetrization of an abundant achiral diol en route to a key four-carbon
building block as a practical alternative to a protected Roche ester
reduction.
The addition of terminal alkynes to racemic β-stereogenic α-keto esters was achieved in high levels of stereoselectivity, affording versatile tertiary propargylic alcohols containing two stereocenters. This environmentally benign enantioconvergent reaction proceeds with perfect atom economy, requires no solvent, and is catalyzed by a non-toxic zinc salt. The alkyne moiety can be leveraged in downstream transformations including hydrogenation to the corresponding saturated tertiary alcohol, which represents the product of a formal enantioconvergent aliphatic nucleophile addition.
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