No abstract

The acetolyses of 3-substituted cyclobutyl tosylates (X = Ar, C1, and OEt) were examined giving rate constants and product distributions. With the alkyl-and aryl-substituted compounds, the rate-determining step leads to the formation of a bridged cyclobutyl cation which then rearranges to a cyclopropylcarbinyl/homoallyl ion. The value of p for the solvolysis of the 3-aryl derivatives was -1.5, suggesting some charge transfer to the 3-position. The 3-chlorocyclobutyl tosylates react to give only the inverted 3-chlorocyclobutyl acetates. The 3-ethoxy derivatives give -1: 1 mixtures of the corresponding acetates. Further information concerning these reactions was obtained via a b initio M O calculations a t the MP2/6-31G* theoretical level. They showed that except for the silyl substituents, all of the other 3-substituents led to a decrease in energy on rearrangement to the corresponding cyclopropylcarbinyl ion. The latter was found to have considerable homoallyl cation character when substituted at the 2-position. All of the results are in accord with the hypothesis that the rate-determining step with X = alkyl or aryl is the formation of a. bridged cyclobutyl cation that is a transition state for a stereospecific rearrangement to the corresponding cyclopropylcarbinyl ion. The reaction products are then formed from the latter ion.

No abstract

We synthesized porcine neuropeptide Y (pNPY) N-terminal fragments by solid-phase synthesis techniques and analyzed them for solution conformational properties by CD and 1H-nmr spectroscopy. The analogues pNPY1-9 and pNPY1-14 displayed CD spectra indicative of random structures and showed no evidence for induced alpha-helical structures in trifluoroethanol (TFE) up to 50%. However, the CD spectra of pNPY1-9 suggested a conformational shift in tetrahydrofuran. Although in aqueous solution the CD spectra of pNPY1-21 indicated random structures with induction of only a small percentage of alpha-helix in aqueous TFE, pNPY1-25 displayed 13% alpha-helical structure in aqueous solution that increased to 40 and 41% by the addition of TFE and methanol, respectively. The nmr spectra of pNPY1-9 and the proline region of pNPY1-25 indicated extended structures with all-trans conformers at Pro5 and Pro8 for pNPY1-9 and at Pro5, Pro8, and Pro13 for pNPY1-25; in each case the Tyr1-Pro2 amide bond was in both cis and trans conformations. However, observed nuclear Overhauser effect correlations and HN exchange experiments indicated an alpha-helical segment in pNPY1-25 initiated by Pro13 and extending from residues 14 to 25. Thus, the N-terminal polyproline region of NPY has no propensity to fold into a regular secondary structure, although Pro13 is a helix initiator, a result consistent with the proposed role of this amino acid in the NPY structural model.

The series of heterocyclic compounds pyridine, phosphorin, arsenin, antimonin, bismin, and the related series of pyridine oxide, etc., and pyridine sulfide, etc., were investigated using density functional theory. In addition to molecular geometries and energies, the nucleus independent chemical shift (NICS) values were calculated for each ring, and the CHELPG atomic charges were calculated. Calculated heats of formation are: pyridine +133, phosphorin +181, arsenin +231, antimonin +329, bismin +468, pyridine oxide +143, phosphorin oxide +1, arsenin oxide +183, antimonin oxide +344, bismin oxide +570, pyridine sulfide +252, phosphorin sulfide +205, arsenin sulfide +328, antimonin sulfide +435, and bismin sulfide + 642 kJ/mol. The question of aromaticity for these heterocycles was discussed. On the basis of hydrogenation energies, NICS values, and lack of bond alternation, the entire series from pyridine to bismin appears to be aromatic and to a roughly equal degree. The situation for the oxides and sulfides of pyridine, phosphorin, and arsenin is much the same, but bismin oxide and bismin sulfide are nonplanar and according to NICS nonaromatic. Antimonin oxide is aromatic by some of the criteria and nonaromatic by others.

The structures of spiropentyl cation and its rearrangement products have been studied via ab initio geometry optimization at the MP2/6-31G* and B3LYP/6-311G* theoretical levels with additional calculations at the MP4/6-311G* level. The cation was calculated to be a transition state and underwent a cyclopropylcarbinyl to cyclobutyl cation rearrangement. This led to the products observed in the nitrous acid deamination of spiropentylamine. A closely related transition state, with the second cyclopropyl ring rotated 20° from the normal Cs geometry, was located. It led to a cyclopropyl cation type ring opening to an allyl cation, which formed the products observed in the solvolysis of spiropentyl chloride. The two transition states have essentially the same energy.

Stationary points in a region of the C6H9+ potential energy surface was explored using the CBS‐QB3 method. The region includes the spiropentylmethyl cation and all three bicyclohexyl bridgehead cations. The relative energies of these bicyclohexyl cations are −54 kJ/mol for bicyclo[2.2.0]hex‐1‐yl cation and −75 kJ/mol for bicyclo[2.1.1]hex‐1‐yl cation, with spiropentylmethyl cation being 0 kJ/mol. Bicyclo[2.1.1]hex‐1‐yl cation is a transition state, while bicyclo[3.1.0]hex‐1‐yl cation is not a stationary point after all, but rearranges without barrier to 2‐methylenecyclopentyl cation. Although spiropentylmethyl cation and spiro[2.3]hex‐4‐yl cation are local minima on the CBS‐QB3 potential energy surface, their existence is questioned. Spiro[2.3]hex‐5‐yl cation has an unusual structure, with the central carbon having five nearest neighbors with interatomic distances between 1.5 and 1.7 Å. The results for rearrangement of the spiropentylmethyl cation are in general agreement with previous solvolysis studies.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.