26532-22-9Relevant articles and documents
Hobbs,Magnus
, p. 856 (1974)
Maksic,Randic
, p. 424 (1970)
Ring-Expansion Induced 1,2-Metalate Rearrangements: Highly Diastereoselective Synthesis of Cyclobutyl Boronic Esters
Abell, Joseph C.,Aggarwal, Varinder K.,Fasano, Valerio,Hari, Durga Prasad
supporting information, p. 5515 - 5520 (2020/04/10)
The broad synthetic utility of organoboron compounds stems from their ready ability to undergo 1,2-migrations. Normally, such shifts are induced by α-leaving groups or by reactions of alkenyl boronates with electrophiles. Herein, we present a new strategy to induce 1,2-metalate rearrangements, via ring expansion of vinylcyclopropyl boronate complexes activated by electrophiles. This leads to a cyclopropane-stabilized carbocation, which triggers ring expansion and concomitant 1,2-metalate rearrangement. This novel process delivers medicinally relevant 1,2-substituted cyclobutyl boronic esters with high levels of diastereoselectivity. A wide range of organolithiums and Grignard reagents, electrophiles, and vinylcyclopropyl boronic esters can be used. The methodology was applied to a short, stereoselective synthesis of (±)-grandisol. Computational studies indicate that the reaction proceeds via a nonclassical carbocation followed by anti-1,2-migration.
Ring-Closing Strategy Utilizing Nitrile α-Anions: Chiral Synthesis of (+)-Norchrysanthemic Acid and Expeditious Asymmetric Total Synthesis of (+)-Grandisol
Fujiwara, Tetsuya,Okabayashi, Tomohito,Takahama, Yuji,Matsuo, Noritada,Tanabe, Yoo
, p. 6018 - 6027 (2018/11/23)
Chiral syntheses of two distinct small cycloalkanes, (1R,3R)-(1Z)-norchrysanthemic acid and (+)-grandisol, were performed by characteristic ring-closing methodologies using carbanions at the α-position of nitriles (nitrile α-anions). (i) (1R,3R)-(1Z)-Norchrysanthemic acid, a highly potent ingredient of synthetic pyrethroid containing a cyclopropane structure, was synthesized from readily available (S)-epoxide derived from 3-methyl-but-2-en-1-ol in 7 steps in 23 % overall yield and with > 98 % ee. This sequence involves a trans-selective cyclopropane formation using the nitrile α-anion of (S)-3-mesyloxynitrile as the key step. The present chiral synthesis was performed with effective stereocontrol of both the chirality in the 1,3-positions on the cyclopropane and the Z-geometry of the propenyl group. (ii) (+)-Grandisol, an insect sex pheromone possessing a characteristic cyclobutane structure, was synthesized from commercially available cyclopropyl methyl ketone (route A) or from commercially available 3-cyanopropylzinc bromide and 1-bromo-1-methylpropene (route B) in 10 or 8 steps in 6 % or 8 % overall yield and with 80 % ee. This sequence involves a Shi asymmetric epoxidation of a trisubstituted olefin and a straightforward Stork-type asymmetric cyclobutane formation with clean SN2 stereoinversion using the nitrile α-anion of the chiral epoxynitrile. The present expedient method is the second asymmetric total synthesis starting from achiral compounds.
