Enantiopure cyclopentenes

RRM of enantiopure cyclopentene 382, induced by commercially available catalyst C, was the key step in Blechert s total synthesis of the bis-piperidine alkaloid (+)-astrophylline (384) [159]. Exposure of metathesis precursor 382 to only 1 mol% C provided within 2 h bicycle 383 in 82% yield (Scheme 75). 

An excellent example of a RCM/ROM domino process is shown in the total synthesis of the piperidine alkaloid (-)-halosaline (6/3-19) by Blechert and coworkers (Scheme 6/3.3) [231]. The key step is the reaction of the enantiopure cyclopentene derivative 6/3-17 to give 6/3-18 with 5 mol% of the catalyst 6/3-13. Further transformations of 6/3-18 led to the natural product 6/3-19. 

Fischer alkenylcarbene complexes undergo cyclopentannulation to alkenyl AT,AT-dimethylhydrazones (1-amino-1-azadienes) to furnish [3C+2S] substituted cyclopentenes in a regio- and diastereoselective way along with minor amounts of [4S+1C] pyrrole derivatives. Enantiopure carbene complexes derived from (-)-8-(2-naphthyl)menthol afford mixtures of trans,trans-cycloipentenes and ds,ds-cyclopentenes with excellent face selectivity [75]. The mechanism proposed for the formation of these cyclopentene derivatives is outlined in Scheme 28. The process is initiated by nucleophilic 1,2-attack of the carbon... 

In the presence of BF3 OEt2, complexes of the type TpRe(CO)(MeIm)(f/ -furan) add Michael acceptors to the coordinated furan through a 1,3-propene dipole (Scheme 43). " " The 1,3-propene dipole undergoes cyclization to yield a cyclopentene ring.MVK, 3-pentene-2-one, cyclopentenone, 2,4-hexadienal, croto-naldehyde, methacrolein and methylenenorbornanone were successfully deployed. When the enantiopure complex of 2,5-dimethylfuran was combined with racemic methylenenorbornanone, a single diastereomer was isolated in 80% ee (Scheme 44). 

Despite these data, the high level of stereoselectivity usually observed in the rearrangement of enantiopure VCPs suggests that the concerted, symmetry-controlled pathway might be operational as well. For exanple, Baldwin s group reported that the rearrangement of trans-methyl-substituted VCP led to four different stereoisomeric cyclopentene products. The... 

Sonawane et al. further utilized the photoinduced VCP-CP rearrangement in a formal s5mthesis of the sex pheromone of the male cotton boll weevil, (ij-grandisol. The toluene-sensitized irradiation of enantiopure (+)-A -carene 69 afforded the annulated cyclopentene ( )-72 in... 

The same strategy was used to synthesize enantiopure cyclohexenones with spi-ro-connected cyclopentenes [159], via two sequential thio-Qaisen rearrangements (Scheme 9.36). 

Enantioselective ring opening of epoxides was attained with (salen)Cr(III) complex (191) [68]. Cyclopentene derivatives (190) were converted with Me3SiN3 to azide-alcohols (192) in 80-90% yields up to 98% ee (Scheme 16.56). Kinetic resolution of racemic styrene oxide was performed in 98% ee. This reaction was applied to practical synthesis of enantiopure cyclic 1,2-aminoalcohols by reduction of the azide products by Pt02-catalyzed hydrogenation [69], to synthesis of cyclopentenone derivatives [70] to formal synthesis of bioactive compound Balanol [71], and to solid-phase synthesis of peptides [72]. 

The first synthesis of an enantiopure N-protected bicycloproline rehes on the Rh-catalyzed intramolecular C(sp )—H amination of carbamate 35. This reaction is the key step to install the quaternary aminated center of the substituted cyclopentene ring. After a large screening of reaction parameters, the desired as-oxazohdinone 36 was obtained in 35% yield, along with the secondary C—H insertion product 37, and small amounts oftheaziridine38 (Scheme 39). It should be noticed that the Ag(I)-catalyzed... 

The enantiopure A-Ts cyclopentene carboxamide 35, obtained by a [2+2] cycloaddition and an enzymatic resolution, could also be a platform to synthesize the antiviral agent ( )-abacavir 38 (Scheme 41.8), which is a nucleoside analog reverse transcriptase inhibitor used in combination... 

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