Synthesis of (±)-pseudoheliotridane by allylboration of 1-pyrroline

Reaction of tricrotylborane with 1-pyrroline proceeds stereoselectively to give (1R ^* ,1′S ^* )-2-(1-methylallyl)pyrrolidine. The latter was converted to the pyrrolizidine alkaloid (±)-pseudoheliotridane through hydroboration-oxidation-intramolecular cyclization. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1450–1452, July, 1998.     

Stereoselective synthesis of the C14-C26 fragment of the cytotoxic macrolide FD-891

A stereoselective synthesis of the C₁₄-C₂₆ fragment of the naturally occurring, cytotoxic macrolide FD-891, is described. Asymmetric Evans aldol reactions and aldehyde Brown allylations are key steps of the synthesis.     

Stereoselective synthesis of (±)-indolizidines 167B and 209D and theirtrans-isomers based on the reductive allylboration of pyridine

A general method for the synthesis of 5-substituted indolizidines based on intramolecular cyclization oftrans- andcis-2-allyl-6-R-1,2,3,6-tetrahydropyridines, obtained from pyridine and triallylborane, has been elaborated. The closure of the five-membered ring is carried out by hydroboration-oxidation followed by cyclization of the resulting δ-amino alcohols in the presence of the Ph₃P−CBr₄−Et₃N system. (Pr₂BH)₂ and Pr₃B are used as the hydroborating reagents, and H₂O₂ in an acid medium is used for the oxidation of 2-[3-(dipropylboryl]-Δ²-piperideines formed. This method has been used for the synthesis of two natural alkaloids: indolizidine 209D (cis-5-hexylindolizidine) and itstrans-isomer were prepared fromcis- andtrans-2-allyl-6-hexyl-1,2,3,6-tetrahydropiridine, respectively; indolizidine 167B andtrans-5-propylindolizidine were synthesized fromcis- andtrans-2,6-diallyl-1,2,3,6-tetra-hydropyridine, respectively. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 971–979, May, 1998.     

Stereoselective synthesis of hyptolide and 6-epi-hyptolide

The first stereoselective syntheses of the naturally occurring, ,β-unsaturated lactone hyptolide 1 and of its nonnatural epimer at C-6 are described. Ethyl l-lactate was the chiral starting material. Key steps of these syntheses were a Brown's asymmetric allylation, a Carreira's asymmetric ethynylation and a ring closing metathesis.     

Highly Diastereoselective Synthesis of Methylenecyclobutanes by Merging Boron‐Homologation and Boron‐Allylation Strategies

A highly diastereoselective synthesis of methylenecyclobutanes possessing a quaternary stereocenter is reported, in which boron homologation of an easily‐generated cyclobutenylmetal species is performed, followed by an allylation reaction. Combining three steps in a one‐pot process further optimized the method, which afforded the expected adducts in excellent yields and stereoselectivity, starting from commercially available 4‐bromobutyne.     

Enantioselective syntheses of (E)-γ,δ-disubstituted homoallylic alcohols via BF3·OEt2-catalyzed aldehyde allylboration and analysis of the origin of E-selectivity: A1,2 allylic strain vs. syn-pentane interaction

Enantioselective allylboration of aldehydes with α-substituted β-methyl allylboronate was reported. By using BF₃·OEt₂ as the catalyst, γ,δ-disubstituted homoallylic alcohols were obtained in good yields with high E-selectivities and enantioselectivities. Transition state analysis revealed that the disfavored transition state suffers from a syn-pentane interaction between the BF₃ catalyst and axially oriented α-substituent of the allylboron reagent. Such a syn-pentane interaction is severe enough to overcome the A^1,2 allylic strain between the β-methyl group and the α-substituent of the boron reagent that is present in the favored competing transition state. Consequently, the reaction proceeded with equatorial placement of the α-substituent to furnish γ-methyl substituted homoallylic alcohols with high E-selectivity.     

Reductive mono- and diallylation of the bis(pyridine)dihydropyridyllithium dimer by triallylborane

Reductive allylation of the bis(pyridine)dihydropyridyllithium dimer containing the 1,2- and 1,4-dihydropyridine fragments by triallylborane results mainly in trans- and cis-2,6-diallylpiperidines (60—85%), their ratio depending on the nature of the solvent. The minor reactions products are 2-allyl-1,2,3,6-tetrahydropyridine and 4,10-diallyl-3,9-diazatricyclo[6.2.2.0^2,7]dodecane. The unexpected formation of the latter is due to hetero-Diels—Alder condensation of intermediate products formed in the allylboration of dihydropyridines. The stereochemistry of trans- and cis-2,6-diallylpiperidines was determined from the ¹H and ¹³C NMR spectra of the respective N,N-dimethylpiperidinium iodides. The structure of 4,10-diallyl-3,9-diazatricyclo[6.2.2.0^2,7]dodecane dipicrate was established by X-ray diffraction analysis.     

A concise and enantioselective approach to the total synthesis of (−)-lasubine I

An efficient, enantioselective total synthesis of (−)-lasubine I (1) has been achieved in an overall 8.8% yield from readily available starting materials. The important features of this approach include the creation of stereogenic centers through two sequential highly stereoselective Roush allylborations and the use of S_N2 cyclization and ring-closing metathesis reactions for the construction of the quinolizidine skeleton.     

Cross‐Metathesis/Isomerization/Allylboration Sequence for a Diastereoselective Synthesis of Anti‐Homoallylic Alcohols from Allylbenzene Derivatives and Aldehydes

We describe a highly diastereoselective approach to anti‐homoallylic alcohols from allylbenzene derivatives and aldehydes. The strategy is based on a cross‐metathesis/isomerization/allylboration sequence catalyzed successively by ruthenium and iridium. This methodology provides another way to access this class of compounds, which leads to the preparation of hitherto‐unknown homoallylic alcohols without the requirement to control the stereochemistry of the 1‐alkenyl boronate intermediates. Our study towards an enantioselective version of this sequential reaction is also reported.     

Stereoselective synthesis of cis-4,7-disubstituted 4,7-dihydroxy-4,7-dihydro[2.2]paracyclophanes

The nucleophilic addition of butyllithium, phenyllithium, methyllithium, and triallylborane to [2.2]paracyclophane-4,7-quinone (1) proceeded regio- and stereospecifically to give the corresponding cis-4,7-disubstituted 4,7-dihydroxy-4,7-dihydro[2.2]paracyclophanes 2—5 with the endo orientation of the hydroxy groups. The structures of quinone 1 and diols 2, 4, and 5 were established by X-ray diffraction analysis.