Oxa‐bis‐ und Oxa‐tris‐σ/π‐homobenzole Synthesen und σ → π‐Isomerisierungen

Abstract

Oxa-bis-and Oxa-tris-u/n-homobenzenes Syntheses and a + n-IsomerhitionsAddition of diazomethane to the benzene oxides 13a-d yields -under the cis-directing influence of the oxirane oxygen atom often stereoselcciively -the cis-mono- (16% c, d), the cis-bis-(17a-c) and the rr.utis-bis-l-pyrazolines 19b. Photolysis of 16a.c.d. of 17b.c. and of 19b proceeds exclusively to the oxa-bis-a-homohenzenes 20a, c, d and the cislrrans-oxa-tris-a-homobenzenes 24b, c and 30b, respectively. In the case of the diester 17a formation of 24. faces competition by H-migration (20e) upon direct excitation, by transannular C -C-bridging (25a) upon acetone-sensitized excitation. The cis-oxa-bis-a-homobenzenes 20a -e thermally isomerize rapidly -and with the exception of 20c -reversibly to the 4H-oxocines 21a-e, the cis-oxatris-a-homobenzenes 24a-c irreversibly to the 4,7-dihydrooxonines 31a-c. In contrast, the equally strained rrans-isomers 30a, b are thermally much more stable and decompose above 180-200°C in an unspecific way. Kinetic and thermodynamic parameters for these 20 + 2n-and 30 + 3n isomerization reactions as well as the spectroscopic data typical of the oxa-bisand oxa-tris-ain-homobenzene structures are presented, the classification of the transformations 24a -c -, 31a-c as [02s + a2s + a2s]-cycloreversions is discussed.

π-Facial stereoselectivity in the Diels–Alder reactions of benzene oxides

“…After warming to room temperature the solvent was evaporated in in vacuo. Recrystallization of the residue from hexane gave 7.59 g (76%) of 6 as colorless needles: mp 82-83°C; ir (KBr pellet) v, , : 1466(KBr pellet) v, , : , 1384(KBr pellet) v, , : , 1322(KBr pellet) v, , : , 1167 6.8,7.7,9.1Hz),2.90(IH,dd,J=4.5,15.4Hz),2.69(1H,dd,J= 6.8,16.1Hz),2.61(1H,dd,J = 7.7,16.1 Hz),2.29(1H,dd,J = 8.3,15.4Hz),1.34(3H,s), and 1.32 (3H, s); I3C nmr 6: 61.9 and 61.0 (oxirane carbons), 50.5 and 49.6(2 X CHBr), 40.8 and39.9(2 X CH2), 19.9and 19.5 (2 X CH3); ms m/z (%): 205 (1 l), 203 (12), 165 (I), 163 (I), 123 (20), 95 (7), 81 (17), 79 ( l l ) , and 43 (100).…”

“…Similarly, adducts derived from 1,2,4-triazoline-3,s-dione dienophiles have been reported as the products of anti addition (IS), but this assignment of stereochemistry was based on one structure that was determined by X-ray crystallography (16). In contrast, on acetone-sensitized excitation, diazomethane added to the face of a benzene oxide syn to the oxygen (17). We report here the Diels-Alder cycloadditions of three benzene oxides with carbon dienophiles, and the first irrefutable proof for the stereochemistry of the products.…”

Seven‐Membered Heterocycles: Azepines, Benzo Derivatives and Related Systems

“…Early studies based on X-ray analysis discarded the azepine-benzeneimine equilibrium (Scheme 21.4) for simple 1H-azepines in the solid state, and 1 H NMR data obtained at different temperatures were also consistent with the azepine structure [58]. However, other NMR studies [88] 84] demonstrated the existence of the azepine-benzeneimine equilibrium in densely substituted 1H-azepines such as 72, where the bicyclic isomer 73 is present at a level of 3-10% in the equilibrium mixture (Scheme 21.5). Further theoretical studies (MINDO/3) showed that the benzeneimine is a nonplanar bicycle in which the aziridine ring is 72.6 out of the plane of the six-membered ring.…”

Three-Membered-Ring-Based Molecular Architectures

“…The principal steps in these elaborate preparations were the stereoselective ring opening of 297 with the weakly basic mixed cyanocuprate of lithiated tris(isopropylsilyl)-protected propyne and fixation of the all-axial orientation of the six substituents on the flexible cyclohexane precursor 298 by bridging the three hydroxy groups in the form of the adamantoid orthophosphates 299 − 301 (Scheme ) . Successful application of this strategy led to the preparation of cis -tris-σ-homobenzenes 302 -iPr, 302 -Me, 304 , and 306 in 15%, 13%, 11%, and 2.5% overall yield, respectively …”