133665-56-2Relevant academic research and scientific papers
Stereochemistry and Mechanism of the Reverse Ene Reaction of cis-2-Alkyl-1-alkenylcyclobutanes. Stereoelectronic Control in a System Showing Marginal Energetic Benefit of Concert
Getty, Stephen J.,Berson, Jerome A.
, p. 4607 - 4621 (2007/10/02)
In the temperature range 243.8-267.5 °C, the racemic cyclobutanes (1RS,2RS,1′SR)-1-(1-methoxyethyl)-2- vinylcyclobutane (16b) and (1SR,2SR,1′SR)-1-(1-methoxyethyl)-2-vinylcyclobutane (17b) each undergo a sigmatropic hydrogen shift (reverse ene reaction) amounting to about 18% of the total pyrolysis product, in addition to four other unimolecular processes. The other four reactions are [2 + 2]-cycloreversion, epimerization, double epimerization, and sigmatropic carbon 1,3-rearrangement. Overall disappearance of reactant occurs with first-order kinetics. The activation parameters determined for 16b are Ea = 47.8 ± 2.1 kcal/mol and log A = 14.8 ± 0.9, and for 17b they are Ea = 48.6 ± 2.2 kcal/mol and log A = 15.2 ± 0.9 (A, s-1). In the reverse ene reactions, vinyl derivatives 16b and 17b yield (2E,6Z)-2-methoxyocta-2,6-diene ((E,Z)-18) and (2Z,6Z)-2-methoxyocta-2,6-diene ((Z,Z)-18), respectively, with stereospecificities of 95 and 91%. These are minimum values because of the competing interconversion of the reactant cyclobutanes 16b and 17b. Correction for this gives a stereospecificity of about 220:1 for the 6Z reverse ene product formed directly from 16b and about 35:1 for that from 17b. This demonstrates high stereospecificity at the double bond of the product derived from the migration origin. Secondary 1,3-deuterium kinetic isotope effects (ΔΔG*), measured at 517 K) for [2 + 2]-cycloreversion, epimerization, and sigmatropic carbon 1,3-rearrangement of 10-20 (±120) and 80-180 (±240) cal/mol were measured for 16b and 17b. Primary isotope effects for reverse ene hydrogen shift were 980 ± 125 and 1760 ± 290 cal/mol, respectively. The stereochemistry at the terminus of migration can be determined in the corresponding cyclobutanes bearing a propenyl instead of a vinyl group. The enantiomerically enriched isotopically labeled cyclobutane (1R,2R,1′S)-(-)-1-(1-methoxyethyl)-2-(2-deuterio-1(E)-propenyl)cyclobutane ((-)-37) having 93.2% ee was prepared in nine steps from (S)-(-)-3-butyn-2-ol. The reverse ene reaction accounts for roughly 5% of the total product in the pyrolysis of the propenyl derivative (-)-37-d at 239.3 °C. The principal reverse ene product is (2E,6Z)-8(S)- deuterio-2-methoxynona-2,6-diene (45), formed with roughly 13:1 stereospecificity compared to the next most prevalent double bond isomer. The product 8(S)-45 was chemically degraded to (S)-2-deuteriopropanoic acid, which was subsequently converted to the corresponding propanoate ester 49/50 with (R)-(+)-methyl mandelate. Analysis of the diastereomeric excess of this ester by 1H and 2H NMR indicates that 8(S)-45 is formed from (1R,2R,1′5)-(-)-37-d with 68.9 ± 4.9% (1H NMR) or 80.5 ± 3.3% (2H NMR) transfer of stereogenicity. Taking into account potential stereochemical contaminants, the actual stereogenicity transfer may be as high as 100% but not lower than 64%. This suffices to show that a suprafacial hydrogen transfer dominates the retro ene reaction in the propenyl case and probably also in the vinyl cases. The results from 16b, 17b, and 37 are interpreted in terms of a dominant concerted mechanism for the reverse ene reactions of these compounds. They are consistent with a transition-state structure in which the alkenyl moiety is endo with respect to the ring and the breaking C-H bond orbital is aligned with the breaking C-C ring bond. This is the sole geometry consistent with predictions based on the conservation of orbital symmetry and orbital overlap control.
Remote Control of Stereogenicity Transfer by Ring-Generated Anisotropic Orbital Overlap. Stereochemistry of Hydrogen Shift in the Intramolecular Reverse Ene Reaction of a cis-2- Alkyl-1 -alkenylcyclopropane
Parziale, Patti A.,Berson, Jerome A.
, p. 4595 - 4606 (2007/10/02)
The thermal rearrangement of cis-2-(2-propyl)-1(E)-propenylcyclopropane at temperatures near 230 °C in the gas phase occurs with activation parameters of Ea = 35.5 ±0.6 kcal/mol and log A = 12.05 ±0.5 (A, s-1). The optically active isotopically doubly labeled analogue (cis-2(S)-(2(S)-propyl-1-d3)-1(S)-(1(E)-propenyl-2-rf)cyclopropane 5 was synthesized in 12 steps from dicyciopentadiene. Pyrolysis of 5 gave only 2-methyl-octa-2(Z),5(Z)-diene-1-d3-7(S)-d, with high stereospecificity at each of the three sites of stereogenicity. This result is the one predicted if the reaction is controlled by optimal overlap ' of the reacting C-H and π bond orbitals with the Cs symmetric component of the degenerate 3E' highest occupied orbital of the cyclopropane ring.
BIOSYNTHESIS OF THE POLYETHER ANTIBIOTIC MONENSIN-A. RESULTS FROM THE INCORPORATIONS OF LABELLED ACETATE AND PROPIONATE AS A PROBE OF THE CARBON CHAIN ASSEMBLY PROCESSES
Sood, Gulshan R.,Ashworth, Doreen M.,Ajaz, Abid A.,Robinson, John A.
, p. 3183 - 3194 (2007/10/02)
The incorporation of sodium - and (S)--propionate into the polyether antibiotic monensin-A in cultures of Streptomyces cinnamonensis occurs with retention of label only at C-4 and C-6, whereas during the incorporation of sodium (R)-propionate the deuterium label is lost to the medium.These results are consistent with the formation of (S)-methylmalonyl-CoA from the labelled propionate by carboxylation of propionyl-CoA with loss of the 2-pro-R hydrogen.The (S)-methylmalonyl-CoA is subsequently incorporated into the antibiotic by a decarboxylative condensation occuring with overall inversion.The incorporations of sodium -and -acetates into monensin-A provide evidence for a pathway of metabolism leading to methylmalonyl-CoA that does not proceed via succinyl-CoA.Instead, acetyl-CoA may be processed via butyryl-CoA and isobutyryl-CoA, to afford (S)-methylmalonyl-CoA.
