Aspidosperma alkaloids synthesis

Racemic Syntheses of Aspidosperma Alkaloids Syntheses of vindoline and several other Aspidosperma alkaloids containing the C(5)-ethyl substituent have been accomplished via intermediacy of C(5)-ethyl substituted substrates similar to 494c (Scheme 16.103). For example, tandem, double... 

The intramolecular cycloaddition reaction of enamides has been exploited in alkaloid synthesis (81JOC3763). One successful application is provided by the total synthesis of the fused indolizidine 5 from 4 as a 1 1 mixture of epimers in 43% total yield 5 is a key intermediate in aspidosperma alkaloid synthesis (79JA3294). 

Synthesis The final stages are very similar to Review Problem 20 (frames 212-3). TM 249 is an intermediate in Stork s synthesis of Aspidosperma alkaloids. Stork s method was actually a variation on the one we have proposed ( J. Amer. Chem. Soc., 1963, 85, 2872) ... 

Snyder has conducted similar chemistry but with the goal of generating carbon skeletons for the total synthesis of alkaloids. Using indole 84 as a dienophile, the canthine alkaloid skeleton 85 was produced. Access to aspidosperma alkaloids was obtained when 86 was transformed into 87. 

Scheme 3.62. Domino radical hydrogen abstraction-cyclization procedure in the synthesis towards Aspidosperma alkaloids.

Since in the synthesis of heterocyclic compounds the ring closure usually involves the formation of the carbon-heteroatom bond, in the retrosynthetic analysis the first bond to be disconnected is the carbon-heteroatom bond (Cf. heuristic principle HP-8), either directly or after the pertinent (FGI or FGA) functional group manipulation. For instance, compound 17 -which is the starting material for Stork s synthesis of Aspidosperma alkaloids [30]- may be disconnected as shown in Scheme 6.11. 

The versatile cyclohexa-1,4-diene 32a has served as an intermediate for synthesis of (—)-eburnamonine 81 and the Aspidosperma alkaloid (—)-aspidospermidine 84 (Scheme 18).3s Butyrolactone carboxylic acids 78 and 82 were prepared from 32 by modification of the methodology outlined in Scheme 7. The key Pictet-Spengler-type cyclization of 79 under conditions of kinetic control gave an 18 1 mixture of 80 and its C(3) (3-epimer in 93% yield. Subsequent hydroboration... 

In more recent work, Chiu and co-workers [167, 168] have reported an intramolecular 1,3-dipolar cycloaddition approach toward the pseudolaric acids 85, in which the di-polarophile is an unactivated 1,1-disubstituted alkene. Hence, treatment of the diazo ketone 86 with catalytic Rh2(OAc)4 furnished a mixture of tricyclic products 87 and 88 in nearly equal proportions (Scheme 19.13). The synthesis of 2-pyridones [169] and their application to the ipalbidine core [170] has been described. The pentacyclic skeleton of the aspidosperma alkaloids was prepared via the cycloaddition of a push-pull carbonyl ylide [171]. The dehydrovindorosine alkaloids 89 have also been investigated, in which the a-diazo-/ -ketoester 90 undergoes a facile cycloaddition to furnish 91 in... 

Padwa and co-workers (60,106,107) have been highly active in using carbonyl ylides for the synthesis of a number of bioactive alkaloids (Scheme 4.51). In an approach to the aspidosperma alkaloids, a push-pull carbonyl ylide was used to generate a bicyclic ylide containing a tethered indole moiety. This strategy ultimately allowed for the synthesis of the dehydrovindorosin skeleton (108). Starting from a quaternary substimted piperidone (200), elaboration of the 3-carboxylic acid provided p-ketoester amide 201. Addition of the indole tethered side chain provided a very rapid and efficient method to generate the cycloaddition precursor 203. 

The A2-piperideines are capable of behaving as dehydrosecodine analogs in alkaloid synthesis. One of the first successful applications of this approach is provided by the total synthesis of the aspidosperma alkaloid minovine (234 Scheme 41) (73JA7146). 

A closer analogy to dehydrosecodine (228) has been developed using A2-piperideines (e.g. 236). These compounds are not isolated but are produced by the fragmentation of salt (235). This elegant method is illustrated in Scheme 42 by the synthesis of racemic pandoline (237) and its C2o epimer. This reaction has provided an efficient route to a number of aspidosperma alkaloids (80JOC3259). 

The decomposition of 368 catalysed by Rh perfluorobutyrate and subsequent intramolecular cycloaddition give 370 in high yield (93%) as the key step in the total synthesis of lysergic acid (371), and is believed to involve the intramolecular reaction of the ylide intermediate 369 at the alkene. No C—H insertion takes place [122], Another elegant example is the efficient construction of the aspidosperma alkaloid skeleton 374. The Rh-catalysed domino cyclization cycloaddition of diazo irnide 372 afforded cycloadduct 374 in 95% yield as a single diastereomer via the dipole 373, and desacetoxy-4-oxo-6,7-dihydrovindorosine (375) has been synthesized from 374 [123]. 

A bromobenzyl moiety is a versatile moiety for triggering radical translocations. When it is attached to a nitrogen atom, the resulting cz-amino radical can be an interesting intermediate for alkaloid synthesis as demonstrated by Jones in an elegant approach to the ABCE-rings of the Aspidosperma and Strychnos families [75]. 

The intramolecular azide cycloaddition has also been used in approaches to the aspidosperma alkaloids <2004TL919, 20050BC213>. The cycloaddition of 123 proceeds directly to aziridine 124 in 80% yield (Equation 28) <2004TL919>. This is an interesting transformation in that none of the initially formed triazoline is observed and because of the high regioselectivity of the addition. A conceptually related approach to the synthesis of cephalotaxine has also been reported <1997TL4347>. 

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