Acid-catalyzed aza-Diels-Alder reactions for the total synthesis of (+/-)-lapatin B

A 5-step total synthesis of microfungal alkaloid (+/-)-lapatin B has been accomplished via a key 2-aza-Diels-Alder reaction. Br?nsted acids catalyze the cycloaddition step and provide improved exo selectivity. This synthetic route has been applied to the construction of related spiro-quinazoline structures.     

Concise stereocontrolled formal synthesis of (+/-)-quinine and total synthesis of (+/-)-7- hydroxyquinine via merged Morita-Baylis-Hillman-Tsuji-Trost cyclization

Concise stereoselective syntheses of (+/-)-quinine and (+/-)-7-hydroxyquinine are achieved using a catalytic enone cycloallylation that combines the nucleophilic features of the Morita-Baylis-Hillman reaction and the electrophilic features of the Tsuji-Trost reaction. Cyclization of enone-allyl carbonate 11 delivers the product of cycloallylation 13 in 68% yield. Diastereoselective conjugate reduction of the enone 13 (>20:1 dr) followed by exchange of the N-protecting group provides the saturated N-Boc-protected methyl ketone 19, which upon aldol dehydration provides quinoline containing enone 15, possessing all carbon atoms of quinine. Exposure of ketone 15 to L-selectride enables diastereoselective carbonyl reduction (>20:1 dr) to furnish the allylic alcohol 16. Stereoselective hydroxyl-directed epoxidation using an oxovanadium catalyst modified by N-hydroxy-N-Me-pivalamide delivers epoxide 17 (17:1 dr). Cyclization of the resulting amine-epoxide 17 provides (+/-)-7-hydroxyquinine in 13 steps and 11% overall yield from aminoacetaldehyde diethyl acetal. Notably, highly stereoselective formation of five contiguous stereocenters is achieved through a series of 1,2-asymmetric induction events. Deoxygenation of the N-Cbz-protected allylic acetate 22 provides olefin 23, which previously has been converted to quinine. Thus, (+/-)-quinine is accessible in 16 steps and 4% overall yield from commercial aminoacetaldehyde diethyl acetal.     

A simple method of synthesis of (+/-)-2,3-diarylpiperazines and a novel method of resolution of (+/-)-2,3-diphenylpiperazine

Intramolecular reductive coupling of diimines in the presence of Zn/Ti(O(i)Pr)2Cl2 gives the corresponding (+/-)-2,3-diarylpiperazines in 73-83% yields with dl/meso ratio >99%:<1%. The (+/-)-2,3-diphenylpiperazine obtained in this way was readily resolved partially using L-(+)-tartaric acid, and the enantiomeric purity was enhanced to >99% ee via preparation of hydrogen-bonded salt aggregates using oxalic acid.     

Formal total synthesis of (+/-)-vindoline by tandem radical cyclization

A formal total synthesis of (+/-)-vindoline 1 has been achieved featuring the tandem cyclization of radicals produced from the iodoaryl azide 19a.     

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.     

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

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

Total synthesis of (+/-)-asteriscanolide.

The total synthesis of asteriscanolide (1) has been achieved by taking advantage on an intermolecular Pauson-Khand cycloaddition and a ring-closing metathesis as key bond-forming transformations. The approach incorporates the cyclooctane stereogenic center prior to ring formation. Interestingly, the ring-closing metathesis generates a new eight-membered ring with an "in-out" intrabridgehead relationship.     

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.     

Total synthesis of (+/-)-scopadulin.

The first total synthesis of (+/-)-scopadulin, an aphidicolane diterpene, is described. The core structure (A/B/C/D ring system) was constructed by an initial synthesis of the B/C/D ring system by our reported methods and a subsequent A ring cyclization by intramolecular aldol condensation. A highly stereoselective cyanation of the tetracyclic enone by Et2AlCN gave a trans-fused A/B ring system with a beta-cyanide at C-4. Stereoselective construction of a quaternary carbon at C-4 was achieved by alpha-alkylation of the cyano group and conversion of the sterically hindered cyano group to a methyl group via our novel reaction for conversion of primary aliphatic amines into alcohols. Finally, the total synthesis of (+/-)-scopadulin was accomplished by a highly chemo- and stereoselective methylation at C-16 and modification of the C-4 alpha-functionality. The stereoselectivity observed in the MeTi(O-i-Pr)3-mediated methylation for the generation of a tertiary axial alcohol at C-16 is extremely high.     

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

[reaction: see text] Herbindole B and cis-trikentrin B are naturally occurring indoles having the unusual and synthetically challenging pattern of carbon substitution at the 4-7 and 5-7 positions, respectively, with no substitution at the 1-3 positions. The total syntheses of these polyalkylated indoles have been achieved in 19 and 12 steps, respectively. The synthesis of herbindole B relies on two iterations of a quinine monoimine Diels-Alder reaction, while cis-trikentrin B uses a single cycloaddition of a suitable quinone monoimine. Indolization of the adducts provides suitably substituted benzopyrrole nuclei for elaboration to the natural products.     

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.     

First total synthesis of (+/-)-taxifolial a and (+/-)-iso-caulerpenyne

The first synthesis of (+/-)-taxifolial A and iso-caulerpenyne was accomplished. The key steps in the sequence are (1) the stereoselective assembly of a vinyltin derived from butynediol and a functionalized aldehyde and (2) the construction of the dienyne moiety via a Stille cross-coupling.