Stereoselective synthesis of tetrahydroisoquinoline alkaloids: (−)-trolline, (+)-crispin A, (+)-oleracein E

Tetrahydroisoquinoline alkaloids, (S)-(−)-trolline, (R)-(+)-crispin A, and (R)-(+)-oleracein E, have been synthesized stereoselectively from the both enantiomers of common intermediate (S)-4 and (R)-4. The key step in the synthesis include a stereoselective Bi(OTf)₃-catalyzed intramolecular 1,3-chirality transfer reaction of chiral non-racemic amino allylic alcohols (S)-6 and (R)-6 to construct both enantiomers of (E)-1-propenyl tetrahydroisoquinoline 4.     

An Enantioselective Approach to the Securinega Alkaloids: The Total Synthesis of (+)-Norsecurinine and (+)-Allonorsecurinine

Total syntheses of (+)-norsecurinine and (+)-allonorsecurinine are described that utilize a rhodium carbenoid-initiated O-H insertion/Claisen rearrangement/1,2-allyl migration domino process for the stereoselective introduction of the tertiary alcohol moiety. Overall the employed strategy is flexible and will allow access to other members of the Securinega family of alkaloids.     

Total synthesis of (+)-condylocarpine, (+)-isocondylocarpine, and (+)-tubotaiwine

The first enantioselective total syntheses of indole alkaloids of the condylocarpine type are reported. (+)-Condylocarpine, (+)-isocondylocarpine, and (+)-tubotaiwine were prepared in high enantiomeric purity (er > 99:1) from (1S,5R)-hexahydro-1,5-methano-1H-azocino[4,3-b]indole-12-one 7b by way of five or six isolated intermediates.     

Synthesis of Aristotelia-Type Alkaloids. Part XIII. Total syntheses of (+)-makonine, (+)-aristotelinone,and (+)-11,12-didehydroaristoteline

The oxidative transformation of synthetic (+)-aristoteline ((+)- 6 ) into other metabolites which had been isolated from Aristotelia species was investigated. Thus, treatment of (+)- 6 with I₂ as the single oxidant furnished the naturally occurring indole alkaloids (+)-makonine ((+)- 9 ),(+)-aristotelinone ((+)- 11 ), or (+)-11, 12-didehydroaristoteline ((+)- 7 ) in good yields, the selectivity of the oxidation process depending on the chosen reaction conditions.     

A new asymmetric synthesis of 2,6-cis-disubstituted 4-methylenepiperidines: total synthesis of (+)-alkaloid 241D and (+)-isosolenopsin A

A highly diastereoselective synthesis of 2,6-cis-disubstituted-4-methylenepiperidines based on a Mannich type intramolecular cyclization of an allylsilane on an iminium ion is described. The synthetic potential of this methodology is demonstrated by the enantioselective synthesis of two natural piperidine alkaloids: (+)-alkaloid 241D and (+)-isosolenopsin A.     

Sedum alkaloids—VII : Structure and synthesis of (+)-4-hydroxysedamine and (+)-4-hydroxyallosedamine

The structures of (+)-4-hydroxysedamine ¹ and (+)-⁴-H-hydroxyallosedamine ², two new minor alkaloids from Sedum itacre, are reported 1 and 2 were synthesized, and their absolute configuration established, via the optically pure tetrahydro 1,3-oxazines 17 and 18.     

The first enantiospecific total synthesis of a C-quaternary voachalotine alkaloid, (+)-dehydrovoachalotine

The first enantiospecific total synthesis of the indole alkaloid (+)-dehydrovoachalotine (1) has been achieved from d-(+)-tryptophan methyl ester in 28% overall yield. The formation of the prochiral quaternary carbon center at C-16 in the key intermediate (12) was realized via a Tollens reaction from N_a-methylvellosimine (13) in 95% yield. This approach could also be applied to the synthesis of many other indole alkaloids that contain a quaternary carbon center at C(16).     

Concise enantioselective total syntheses of (+)-homochelidonine, (+)-chelamidine, (+)-chelidonine, (+)-chelamine and (+)-norchelidonine by a Pd II-catalyzed ring-opening strategy

New enantioselective syntheses of the B/C hexahydrobenzo[c]phenanthridine alkaloids (+)-homochelidonine, (+)-chelamidine, (+)-chelidonine, (+)-chelamine, and (+)-norchelidonine are described. Our rapid and convergent route to this class of natural products involved the development and application of a Pd II-catalyzed asymmetric ring-opening reaction of a meso-azabicyclic alkene with an aryl boronic acid as the key step. By screening a variety of functionalized ortho-substituted aryl boronic acids, chiral ligands and reaction conditions we were able to prepare the requisite cis-1-amino-2-aryldihydronaphthalenes in high yield and in up to 90 % ee. Early attempts to complete the synthesis of (+)-homochelidonine using an N-Boc azabicyclic alkene are described in full. The successful route required a protecting group alteration followed by B ring formation and then stereoselective installation of the C-11 syn-hydroxy group by regioselective epoxide ring-opening using a hydride source. Ring-opening of the same epoxide intermediate with water ultimately led to the synthesis of (+)-chelamidine. The same strategy was then used to synthesize the other structurally similar B/C hexahydrobenzo[c]phenanthridine alkaloids, (+)-chelidonine, (+)-chelamidine, and (+)-norchelidonine.     

Stereoselective synthesis of (+)-(8R,8aR)-perhydro-8-indolizidinol

A highly stereoselective total synthesis of (+)-(8R,8aR)-perhydro-8-indolizidinol is described. Key steps involved in this synthesis are diastereoselective zinc allylation, azido-olefin cyclization and reductive amination followed by cyclization which effectively constructed the indolizidine ring. This contributes a unique approach to the synthesis of indolizidine alkaloids that offers the advantages of brevity and relatively high overall yields.     

Total syntheses of (-)-haemanthidine, (+)-pretazettine, and (+)-tazettine

[structures: see text] The total syntheses of the amaryllidaceae alkaloids haemanthidine, pretazettine, and tazettine as optically pure enantiomers are reported. Using D-mannose as the starting material, the critical relative stereochemical relationships are established with an intramolecular nitrone-alkene cycloaddition reaction. The synthetic route leads successively to (-)-haemanthidine and then to (+)-pretazettine and (+)-tazettine, taking advantage of the well-established complex relationships among these three alkaloids.     

Enantioselective total synthesis of (+)-lysergic acid, (+)-lysergol, and (+)-isolysergol by palladium-catalyzed domino cyclization of allenes bearing amino and bromoindolyl groups

Enantioselective total synthesis of the biologically important indole alkaloids (+)-lysergol, (+)-isolysergol, and (+)-lysergic acid is described. Key features of these total synthesis include (1) a facile synthesis of a chiral 1,3-amino alcohol via the Pd(0)- and In(I)-mediated reductive coupling reaction between L-serine-derived 2-ethynylaziridine and formaldehyde; (2) the Cr(II)/Ni(0)-mediated Nozaki-Hiyama-Kishi (NHK) reaction of an indole-3-acetaldehyde with iodoalkyne; and (3) Pd(0)-catalyzed domino cyclization of an allene bearing amino and bromoindolyl groups. This domino cyclization enabled direct construction of the C/D ring system of the ergot alkaloids skeleton, as well as the creation of the C5 stereogenic center with transfer of the allenic axial chirality to the central chirality.     

Enantioselective synthesis of (+)-trypargine and (+)-crispine E

Asymmetric transfer hydrogenation was used as a key step in the synthesis of two guanidine-derived alkaloids (+)-trypargine and (+)-crispine E. The enantiomeric compositions of key intermediates were established on the basis of ¹H NMR spectra with chiral solvating agents. The absolute stereochemistry was proven by X-ray crystallography.     

Total synthesis of marine bisindole alkaloids, (+)-hamacanthins A, B and (−)-antipode of cis-dihydrohamacanthin B

The total synthesis of the marine bisindole alkaloids, (+)-hamacanthins A (2a) and B (2b), and (−)-antipode of cis-dihydrohamacanthin B (2e) was achieved by transamidation-cyclization of N-(2-aminoethyl)-2-oxoethanamide 18 derived from (S)-indolylglycinol 10.     

Concise asymmetric synthesis of (−)-halosaline and (2R,9aR)-(+)-2-hydroxy-quinolizidine by ruthenium-catalyzed ring-rearrangement metathesis

A ruthenium-catalyzed ring opening-ring closing metathesis reaction serves as the key step in the stereoselective synthesis of a new enantiopure 2-substituted-4,5-dehydropiperidine skeleton, a valuable intermediate for the synthesis of piperidine alkaloids (such as (−)-halosaline) and of hydroxylated quinolizidines (such as (2R,9aR)-(+)-2-hydroxy-quinolizidine).     

Über die Synthese der Pilocarpus-Alkaloide Isopilosin und Pilocarpin,sowie die absolute Konfiguration des (+)-Isopilosins

A novel synthesis for rac-pilosinine ( 6 rac) and transformation of (+)-pilosinine ( 6 ) into the pilocarpus alkaloids (+)-isopilosine ( 9 ) and (+)-pilocarpine ( 18 ) are described. By correlation via 5 with 18 the absolute configuration of (+)-isopilosine ( 9 ) is established.     

A concise synthesis of (+)-goniofufurone, (+)-7-epi-goniofufurone, (+)-crassalactones B and C

The concise and efficient synthesis of (+)-goniofufurone, (+)-7-epi-goniofufurone, (+)-crassalactones B and C were achieved in 6 to 7 steps from the known d-glucono-δ-lactone derivative 9. An acid-mediated cascade cyclization was employed to construct the furanofurone bicyclic framework in one-pot.     

The enantiospecific,stereospecific total synthesis of the ring-A oxygenated sarpagine indole alkaloids (+)-majvinine, (+)-10-methoxyaffinisine,and (+)-N(a)-methylsarpagine,as well as the total synthesis of the alstonia bisindole alkaloid macralstonidine

The first stereospecific, enantiospecific total synthesis of the ring-A oxygenated sarpagine indole alkaloids (+)-N(a)-methylsarpagine (8), (+)-majvinine (14), and (+)-10-methoxyaffinisine (49), as well as the first total synthesis of the Alstonia bisindole alkaloid macralstonidine (9), has been accomplished. This approach employed the Sch?llkopf chiral auxiliary for the stereospecific construction of the desired d-(+)-tryptophan unit required for the asymmetric Pictet-Spengler reaction. In addition, the strategy was doubly convergent for the enolate-mediated Pd(0) coupling process and the asymmetric Pictet-Spengler reaction can be employed to synthesize both macroline (2) and N(a)-methylsarpagine (8), the coupling of which provides macralstonidine (9). This approach to ring-A substituted alkoxyindole alkaloids should find wide application for the synthesis of other alkaloids for it is stereospecific and either enantiomer can be prepared with ease.     

Stereoselective construction of the pyrrolizidine bridgehead stereochemistry by the adjacent hydroxyl group in the synthesis of (+)-heliotridine and (−)-retronecine

Formal total synthesis of (+)-heliotridine (4) and total synthesis of (−)-retronecine (5) were accomplished by using (S)-3-acetoxysuccinimide (6) as the common starting material. The stereogenic center of 6 ended up as C-1 in both alkaloids. The chiral centers at C-7a of the alkaloids were stereoselectively constructed through the help of the adjacent functionality at C-1. The B-rings of the alkaloids were formed through α-sulfonyl radical cyclizations.     

Enantioselective Catalysis Coupled with Stereodivergent Cyclization Strategies Enables Rapid Syntheses of (+)‐Limaspermidine and (+)‐Kopsihainanine A

Enantioselective Pd‐catalyzed allylic alkylations of dihydropyrido[1,2‐a ]indolone (DHPI) substrates were used to construct the C20‐quaternary stereocenters of multiple monoterpene indole alkaloids. Stereodivergent Pictet–Spengler and Bischler–Napieralski cyclization/reduction cascades furnish the cis‐ and trans ‐fused azadecalin subunits present in Aspidosperma and Kopsia alkaloids, respectively, en route to highly efficient syntheses of (+)‐limaspermidine and (+)‐kopsihainanine A.     

Palladium-catalyzed asymmetric allylic substitution of 2-arylcyclohexenol derivatives: asymmetric total syntheses of (+)-crinamine, (-)-haemanthidine, and (+)-pretazettine

Much interest has been shown in Amaryllidaceae alkaloids as synthetic targets due to their wide range of biological activities. Over 100 alkaloids have been isolated from members of the Amaryllidaceae family; most of them can be classified into eight skeletally homogeneous groups. We have succeeded in the first asymmetric total syntheses of the crinane-type alkaloids (+)-crinamine (1), (-)-haemanthidine (2), and (+)-pretazettine (3). The starting cyclohexenylamine 14 was obtained from allyl phosphonate 11c by palladium-catalyzed asymmetric amination in 82% yield and with 74% ee. The product was recrystallized from MeOH. Interestingly, (-)-14 with 99% ee was obtained from the mother liquor (74% recovery). Intramolecular carbonyl-ene reaction of (-)-10 proceeds in a highly stereoselective manner to give hexahydroindole derivative 9 as the sole product. In the Lewis-acid-catalyzed carbonyl-ene reaction, an interesting rearrangement product, 20, was isolated in high yield. From 9, (+)-crinamine was synthesized. Thus, the asymmetric total synthesis of (+)-crinamine was achieved in 10 steps from 11c, and the overall yield is 19%. The total synthesis of (-)-haemanthidine was also achieved from 9 by a short sequence of steps.