Synthesis of Functionalised Piperidines Through a [3 + 3] Cycloaddition Strategy

Organo-Catalytic Synthesis of 1,3-Thiazole Derivatives

An efficient synthesis of 1,3-thiazolidine derivatives has been achieved in which 2-pyridinecarboxaldehyde oxime was employed as a base catalyst to effect cyclisation between aziridines and CS2 in DMF at 40 °C. A similar reaction between aziridines and thiocyanates yielded 1,3-thiazoline-2-imine derivatives

Concise synthesis of (+)-allo-kainic acid via MgI2-mediated tandem aziridine ring opening-formal [3 + 2] cycloaddition

3-Methyl vinyl aziridine undergoes a mild MgI2-promoted S(N)2' ring opening and concomitant cyclization with fumarate Michael acceptors to give trisubstituted pyrrolidines. The process is efficient and highly diastereoselective. This methodology has been applied to a concise asymmetric synthesis of (+)-allo-kainic acid.

Synthesis of stereodefined piperidines from aziridines and their transformation into conformationally constrained amino acids, amino alcohols and 2,7-diazabicyclo[3.3.1]nonanes

2-(2-Cyano-2-phenylethyl)aziridines were converted into novel cis- and trans-2-chloromethyl-4-phenylpiperidine-4-carbonitriles via alkylation with 1-bromo-2-chloroethane followed by microwave-assisted 6-exo-tet cyclization and regiospecific ring opening. The latter piperidines were used as eligible substrates for the synthesis of stereodefined 2-chloromethyl-, 2-hydroxymethyl-, and 2-carboxymethyl-4-phenylpiperidine-4-carboxylic acids, 2-hydroxymethyl-4-phenylpiperidine-4-carbonitriles, 3-hydroxy-5-phenylazepane-5-carbonitriles, and 5-phenyl-2,7-diazabicyclo[3.3.1]nonanes.

Investigation of a flexible enantiospecific approach to aziridines

A flexible approach to functionalized and enantioenriched aziridines has been developed from an intermediate aziridinylmethyl tosylate. This protocol features a Cu-catalyzed Grignard substitution reaction that allows a range of functionalized organic fragments to be incorporated. Moreover, a convenient one-pot procedure is outlined that allows the trityl group to be exchanged for a range of common N-protecting groups.

A [3 + 3] Annelation Approach to (+)-Rhopaloic Acid B

A general and enantiospecific [3 + 3] reaction toward functionalized pyrans is reported that has been employed in the first enantioselective synthesis of (+)-rhopaloic acid B.

Pd-catalysed [3 + 3] annelations in the stereoselective synthesis of indolizidines

A [3 + 3] annelation of enantiomerically pure aziridine 7 provides the functionalised piperidine 8 that can be elaborated to the indolizidine skeleton in only 4 steps with good stereocontrol.

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

A [3+3] Annelation Approach to Tetrahydropyridines

[reaction: see text] A stepwise [3+3] annelation sequence to tetrahydropyridines via addition of the Büchi Grignard to aziridines has been developed. These intermediates can be further functionalized with good regio- and stereocontrol and this methodology has been employed in the stereoselective formal synthesis of (-)-dihydropinidine.

Palladium-Catalyzed [3 + 3] Cycloaddition of Trimethylenemethane with Azomethine Imines

A palladium-catalyzed [3 + 3] cycloaddition of trimethylenemethane (TMM) with azomethine imines has been developed to produce hexahydropyridazine derivatives under simple and mild conditions. The use of substituted TMM precursors highlights the difference of this system from previously reported [3 + 2] cycloaddition of TMMs under palladium catalysis. The present [3 + 3] cycloaddition reactions are also applicable to couplings with nitrones.

Development of a Stepwise [3 + 3] Annelation to Functionalized Piperidines

[reaction: see text] A stepwise formal [3 + 3] cycloaddition sequence via a Grignard addition-cyclization reaction leads to a much improved piperidine synthesis. This methodology provides improved flexibility in both the aziridine substrate and TMM equivalent.

Total syntheses of enantiomerically enriched R-(+)- and S-(–)-deplancheine

Total syntheses of indoloquinolizidine alkaloid (+/-)-, R-(+)-, and S-(-)-deplancheine are described here. The synthesis features an enantioselective intramolecular formal aza-[3 + 3] cycloaddition for the construction of the quinolizidine CD-ring. This application serves to introduce a new synthetic strategy for the synthesis of indoloquinolizidine alkaloids.

[3 + 3] Cycloadditions and related strategies in alkaloid natural product synthesis

In this article we describe a strategically unusual approach to piperidines via formal [3 + 3] cycloaddition reactions. The scope and limitations of these processes are outlined and their employment in the synthesis of alkaloid natural products is delineated.

Development of a Flexible Approach to Nuphar Alkaloids via Two Enantiospecific Piperidine-Forming Reactions

In this paper we describe the stereoselective synthesis of functionalized lactam 7 via two enantiospecific piperidine-forming techniques and its employment in a general synthetic approach to Nuphar alkaloids. Specifically, the formation of piperidine 18 by formal [3 + 3] cycloaddition and stepwise annelation processes is described; the latter technique was found to be significantly more efficient than the Pd-catalyzed TMM addition process. Finally, exploitation of the exocyclic alkene installed in the piperidine-forming reaction in the transformation of 18 to (-)-deoxynupharidine ((-)-2), (-)-castoramine ((-)-3), and (-)-nupharolutine ((-)-4) via intermediate lactam 7 is delineated.

A concise asymmetric route to Nuphar alkaloids. A formal synthesis of (-)-deoxynupharidine

[reaction: see text] A stereocontrolled route to Nuphar alkaloids is described that employs a formal [3 + 3] cycloaddition strategy to assemble the piperidine nucleus. The addition of Pd-TMM complexes to aziridine 10 was found to be sluggish; however, the addition of a functionalized allyl Grignard reagent followed by a Mitsunobu condensation reaction provided 11 in high yield. The employment of this route in the formal synthesis of (-)-deoxynupharidine 1 is described.

Additions of allenyl/propargyl organometallic reagents to 4-oxoazetidine-2-carbaldehydes: novel palladium-catalyzed domino reactions in allenynes

Metal-mediated carbonyl allenylation and propargylation of 4-oxoazetidine-2-carbaldehydes were investigated in aqueous environment. Different propargyl bromide and metal promoters showed varied regio- and stereoselectivities on product formation. In addition, an unprecedented one-pot stereoselective synthesis of beta-chlorinated allylic alcohols, which can also be considered as functionalized allylsilanes, has been developed, which involves tin(IV) chloride-mediated reaction of propargyltrimethylsilane and 4-oxoazetidine-2-carbaldehydes. Some of the resulting coupling products were submitted to transition metal catalyzed reactions, such as the allenic Pauson-Khand and palladium-catalyzed reactions, leading to novel fused or bridged tricyclic beta-lactams. Remarkably, a novel domino process, namely the allene cyclization/intramolecular Heck reaction was found. A likely mechanism for the cascade reaction should involve an intramolecular cyclization on a (pi-allyl)palladium complex and a Heck-type reaction.