Base-Promoted Isomerizations

Opening by trimethylsilyl trifluoromethanesulfonate yields an adduct (54) from which trifluoromethanesulfonic acid can be eliminated to give an allylic alcohol (Scheme 47) (79JA2738) [cf. base-promoted isomerization to allylic alcohols (Section 5.05.3.2.2)]. 

Carbamates (R2NCOOR ) may be O or N bonded. Base-promoted isomerization of carbamates from the O- to the deprotonated A--bound form in pentaamminecobalt(III) complexes have been defined.986... 

Base-promoted isomerization of 2,3-epoxy alcohols. Also known as epoxide migration. 

Nevertheless, whereas the base-promoted isomerization of simple linear oxiranes and cyclohexene oxide occurs via a -deprotonation mechanism, recent denterinm-labeUng experiments demonstrate that the LDA-mediated rearrangement of cyclopentene oxide in nonpolar solvents furnishes the corresponding cyclopentenol through an a-deprotonation route (Scheme 7) . 

Based on this feature, aggregation states of transition-state structures for base-promoted isomerization of oxiranes have been established by kinetic studies of LDA-mediated isomerizations of selected oxiranes in nonpolar media in the presence of variable concentrations of coordinating solvents (ligands). Results reported provide the idealized rate law V = [ligand]" [base] [oxtrane] for a-deprotonation and v = fc[ligand]°[base] / [oxirane]... 

The RLi homochiral ligand complexes are seldom used for the base-promoted isomerization of oxiranes into allylic alcohols because their poor chemoselectivity lead to complex mixtures of products. As examples, the treatment of cyclohexene oxide by a 1 1 i-BuLi/(—)-sparteine mixture in ether at low temperature provides a mixture of three different products arising respectively from -deprotonation (75), a-deprotonation (76) and nucleophilic addition (77) (Scheme 32) . When exposed to similar conditions, the disubstituted cyclooctene oxide 78 affords a nearly 1 1 mixture of a- and -deprotonation products (79 and 80) with moderate ee (Scheme 32, entry 1). Further studies have demonstrated that the a//3 ratio depends strongly on the type of ligand used (Scheme 32, entry 1 vs. entry 2) . ... 

Both dienic enamide 220a and its homolog 220b (obtained by base-promoted isomerization of the respective propargylamides 219a and b) afford in a tandem RCM mixtures of 1,3-dienic lactams 221 and 222, though in different proportions (02OL2417). 

Scheme 10 Base-promoted isomerization of propargyi amide foiiowed by RCM.

J. K. Crandall, M. Apparu, Base-Promoted Isomerizations of Epoxides, Org. React. 1983, 29, 345—443. A. C. Cope, E. R. Trumbull, Olefins from Amines The Hofmann Elimination Reaction and Amine Oxide Pyrolysis, Org. React. 1960, 11, 317-493. 

J. K. Crandall and M. Apparu, Base-promoted isomerizations of epoxides, Org. React 1983,29, 345-443. 

Base-promoted isomerizations of oxiranes by way of oxirane C—H abstraction are less common. Lithium tetramethylpiperidide (LiTMP) has successfully been used for this purpose <94CC2103>. Monosubstituted oxiranes are isomerized with LiTMP to aldehydes. 

A mixture of o-menthanes resulted from reduction of o-cymene with Ca(NH3)6,297 and a neat route to the o-menthane skeleton involved photochemical addition of allene to 3-methylcyclohex-2-en-l-one and opening of the cyclobutyl ring with BF3.298 O-Menthadi- and tri-enes resulted from pyrolysis of verbenene.299 Acid-and base-promoted isomerizations of o-menthadienes300 and dehydration of cis- and trans- o-menthan-8-ols with a variety of reagents have been recorded.301 Optically active 2-methyl-4-isopropenylcyclohexanone (which is a useful precursor for ra-menthane derivatives) has been prepared by pyrolysis of chiral 2,2,5-trimethyl-bicyclo[3.1.1 ]heptan-2-one.302... 

Base-Promoted Isomerizations of Epoxides Jack K. Crandall and Marcel Apparu... 

The novel phospha-alkyne system (303) is formed in the reaction of lithium bis(trimethylsilyl)phosphide with 0,0 -diethyldithiocarbamate. C-chlorophosphaethyne, ClCsp, has been generated by the pyrolysis of trichloromethyldichlorophosphine over granulated zinc at 550°C, and characterised by infrared spectroscopy. A bimolecular proton transfer mechanism has been suggested for the base-promoted isomerism of alkynyl- and alkenyl-phosphines to the phospha-alkynes (304). The potential of phospha-alkynes as novel building blocks in heterocyclic chemistry has been reviewed. New examples of phospha-alkyne-derived... 

Olefins, including medium-ring cyclic olefins, have been epoxidized using a polymer supported peracid, thus cis-cyclo-octene was epoxidized in 95% yield. Agerol (293) has been converted into ageratriol (294) by two routes, epoxidation and base promoted isomerization, and photosensitized oxidation followed by sodium boro-hydride reduction. ... 

Similarly, a single DA reaction of a l,2-cyclo[3]dendralene was reported by Kanematsu and coworkers in 1986. In this case, a base promoted isomerization of alkyne 100-generated [3]dendralene 101, which subsequently reacted in an intramolecular DA reaction to yield tricycle 102 (Scheme 12.21) [26]. 

Scheme 1-197. Base-promoted isomerization of bicyclo[2.2.1]hexenols 2S0a and bicyclo[2.2.2]heptenols 250b under C-C bond cleavage.

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