Novel synthesis of indolizidines and quinolizidines

A very short synthesis of indolizidines, quinolizidines and some higher homologues was developed by alkylation of 2-methyl-1-pyrroline or 6-methyl-2,3,4,5-tetrahydropyridine with 1,3- or 1,4-dihaloalkanes, followed by reduction of the intermediate iminium salts, resulting in the desired 1-azabicyclo[m.n.0]alkanes in good yields.     

Total synthesis of (+/-)-calcaridine A and (+/-)-epi-calcaridine A

The first total synthesis of the Leucetta alkaloid calcaridine A is described based on a biosynthetic postulate. Application of an oxidative rearrangement of a 4,5-disubstituted imidazole leads to the formation of both calcaridine A and epi-calcaridine A. An X-ray crystal structure determination on the latter has allowed the assignment of the relative configuration of the epimeric natural product and calcaridine A by extrapolation.     

A stereospecific synthesis of chiral 1,3-disubstituted bromoallenes

Optically active 1, 3-disubstituted propargyl alcohols undergo S_Ni′ rearrangements with thionyl bromide in the presence of propylene oxide to yield bromoallenes without loss of optical purity.     

Biomimetic synthesis of (+/-)-pinnatal and (+/-)-sterekunthal A

Concise biomimetic syntheses of the antimalarial naphthoquinones (+/-)-pinnatal and (+/-)-sterekunthal A are described.     

Total synthesis of (+/-)-herbindole A, (+/-)-herbindole B, and (+/-)-cis-trikentrin A

Herein we describe a divergent total synthesis of the title compounds utilizing Diels-Alder reactions of monoimine quinoids, followed by cyclization of the aromatized adducts to generate the tricyclic skeletons. Elaboration to iodinated or triflated indole derivatives allows for installation of the requisite alkyl substitution via cross-coupling reactions.     

A synthesis of (+/-)-sparteine

In a synthesis of racemic sparteine, Diels-Alder reaction between dimethyl bromomesaconate 14 and dicyclopentenyl 4, followed by cyclopropane formation, set up the stereochemistry at C-1 and C-5 as S and R, respectively, in a meso intermediate 8. The stereochemistry at C-2 and C-4 was then secured by a moderately diastereoselective protonation of the bis-enolate 17 derived from the diester 8 by reductive cleavage with lithium in liquid ammonia. The C=C in the racemic diester 19 was ozonolysed and the diketone converted by Beckmann rearrangement into the bis-lactam . Reduction of the bis-lactam with lithium aluminium hydride and intramolecular nucleophilic displacement gave racemic sparteine 1. Some ideas for making this synthesis amenable to a synthesis of enantiomerically enriched sparteine are presented.     

A synthesis of (+/-)-sparteine

(+/-)-Sparteine has been synthesised with stereochemistry controlled in such a way as to make the route amenable to an efficient synthesis of either enantiomer.     

Total synthesis of (+/-)-didehydrostemofoline (asparagamine A) and (+/-)-isodidehydrostemofoline

The first total syntheses of (+/-)-didehydrostemofoline (1) and (+/-)-isodidehydrostemofoline (3) are reported. The synthesis begins with the Diels-Alder reaction of readily available pyrrole 9 and ethyl (E)-3-nitroacrylate, the latter serving as a regioinverted equivalent of ketene. After hydrogenation to prevent retro-Diels-Alder reaction, the major cycloadduct is transformed to 7-azabicyclo[2.2.1]heptanol 14. Aza-Cope-Mannich reaction of the formaldiminium derivative of 14 delivers 1-azatricyclo[5.3.0.04.10]decane 15, which in 15 additional steps is converted to 1 and 3.     

A formal total synthesis of (+/-)-halichlorine and (+/-)-pinnaic acid

[reaction: see text] A stereocontrolled approach for the preparation of the Danishefsky intermediates has been developed starting with the azaspirobicyclic ketone as a common precursor, representing a formal total synthesis of (+/-)-halichlorine and (+/-)-pinnaic acid. This approach involves the construction of the 1,7-disubstituted 6-azaspiro[4.5]decane with the proper stereochemistry established by olefin hydrogenation followed by C-methylation of the spirotricyclic lactam and the subsequent processes involving lactam ring-opening using methyl triflate and RCM to form the azaspirotricyclic quinolizidine skeleton.     

A convenient synthesis of 1,4-disubstituted imidazoles

An efficient method for the regioselective protection of 4-alkyl-, 4-iodo, 4-vinylimidazoles has been developed via an alkylation-isomerization sequence with various imidazole-protecting groups. The initially formed mixture of 4/5-alkyl-, 4/5-iodo, and 4/5-vinylimidazoles are isomerized to the corresponding 4-isomers on treatment of a DMF solution with a small amount of RX and heating at 75 °C.     

Cascade iminium ion reactions for the facile synthesis of quinolizidines. Concise syntheses of (+/-)-epilupinine and (-)-epimyrtine

Several novel cascade processes have been designed and developed that involve sequential reactions of imines and iminium ions to form substituted quinolizidine ring systems in a single step from simple and readily available starting materials. The utility and promise of these cascade reactions is evident from their application to extraordinarily concise syntheses of the representative quinolizidine alkaloids (+/-)-epilupinine and (-)-epimyrtine.     

Total synthesis of (+/-)-terpestacin and (+/-)-11-epi-terpestacin

The total synthesis of racemic terpestacin and 11-epi-terpestacin using the allene ether Nazarov reaction as the key step is described. All stereochemistry is derived from the stereogenic carbon atom that is formed during the Nazarov reaction.     

First total synthesis of (+/-)-stemonamide and (+/-)-isostemonamide

[structure: see text] The total synthesis of the tetracyclic alkaloids stemonamide (1) and isostemonamide (2) is presented. The key step is the reaction between a silyloxyfuran and an N-acyliminium ion. The second quaternary center is created by an intramolecular aldol spirocyclization. After 1,4-addition of an appropriate side chain, the methyl and double bond are installed by Mannich reaction. The seven-membered ring is closed by intramolecular nucleophilic displacement.     

A concise synthesis of (+/-)-cacalol

A simple synthesis of the natural product cacalol has been developed that proceeds in seven steps and 21-25% overall yield. Ortho-lithiation of 4-methylanisole and alkylation with 5-iodo-1-pentene, followed by intramolecular Friedel-Crafts alkylation, gave 5-methoxy-1,8-dimethyltetralin. This compound was then formylated in the 6-position. Baeyer-Villiger oxidation and hydrolysis of the resulting formate gave 6-hydroxy-5-methoxy-1,8-dimethyltetralin. Alkylation of the phenolic hydroxyl group with chloroacetone followed by cyclodehydration gave cacalol methyl ether. Deprotection of this aryl methyl ether yielded cacalol.     

Total synthesis of (+/-)-merrilactone A

The total synthesis of racemic merrilactone A (a neurotrophic agent) is described, featuring simultaneous and stereospecific creation of the C4 and C5 stereocenters via a notable silyloxyfuran Nazarov cyclization. Full details of the successful synthetic strategy are given, as well as several examples of the interesting reactivity of intermediates that were prepared and studied during the execution of the total synthesis. A detailed investigation of the Lewis acid-catalyzed Nazarov cyclization of silyloxyfurans was conducted, including a systematic study of substrate scope and limitations. In addition, experiments were conducted that suggest the participation of Lewis acidic silicon species in the Nazarov cyclization.     

A two-directional synthesis of (+/-)-perhydrohistrionicotoxin

An entirely two-directional synthesis of (+/-)-perhydrohistrionicotoxin is presented, utilizing a tandem oxime formation/Michael addition/[3 + 2] cycloaddition as the key step. This approach also constitutes formal syntheses of (+/-)-histrionicotoxin and (+/-)-histrionicotoxin 235A.     

Formal synthesis of (+/-)-guanacastepene A

A 17 step synthesis of 55, a late intermediate in Danishefsky's guanacastepene A synthesis, has been completed in 4% overall yield. Key features include the use of vinylmagnesium bromide in the Pd-catalyzed coupling with triflate 13 to give triene 16 without the formation of Heck products, a novel extension of the Stork-Jung vinylsilane Robinson annulation that provides tricyclic 2-hydroxymethylcyclohexenone 42 from 23b in four steps and 51% yield, the ability to obtain almost exclusively alpha'-alkylation of 35ba by the proper choice of protecting groups, and the ability to obtain the desired beta-alcohol selectively by reduction of keto alcohol 42 rather than keto ester 53.