Photorearrangement of some pyrazinobarrelenes

Abstract

Irradiation of pyrazinobarrelene (1) resulted in di-x-methane rearrangement via vinyl-vinyl and pyrazino-vinyl bridges, in the ratio 47 : 53, while irradiation of dicyanopyrazinobarrelenes (2) and (3) afforded only the pyrazinovinyl bridged products.

Photolysis of Fused Cyclobutatriazolines: An Adventitious Route to 3,6-Di(2-pyridyl)pyridazonorbornadiene

“…10 The triazoline (18a) was prepared as outlined in Scheme 3, starting from dimethyl tricyclo[4.2.1.0 2,5 ]nona-3,7-diene-3,4-dicarboxylate (Smith's diene) 11 (13a). Reaction of s-tetrazine (8) with (13a) occurred site-specifically at the norbornene π-bond to form the dihydropyridazine (14a), which was converted to the dpp ligand (16a) by dehydrogenation with ddq (15). 8 Cycloaddition of benzyl azide (17) to the norbornene-1,2-diester π-bond of (16a) was achieved at high pressure (8 kbar) to afford the triazoline (18a) in 86% yield.…”

Synthetic Aspects of the Di-π-methane Rearrangement

“…164 The competition between the di-π-methane and the aza-di-π-methane rearrangements has also been studied in the photochemistry of the pyrazine derivatives of barrelene 261a,b. 172 Direct or acetophenonesensitized irradiation of these brings about the formation of pyrazinosemibullvalenes in a typical di-πmethane process (Scheme 43). Thus, compound 261a gives 262a (53%), by a DPM path, and two other products 263a (25%) and 264a (22%), resulting from an ADPM rearrangement.…”

Diels−Alder Reactions of 4-Triflyloxy-2,6,6-trimethyl-2,4-cyclohexadienone. An Expedient Methodology for the Synthesis of Bicyclo[2.2.2]oct-5-en-2-ones and Bicyclo[2.2.2]octa-5,7-dien-2-ones

“…The Diels−Alder reaction is a widely used protocol in organic synthesis since it generates a wide variety of polyfunctionalized cyclic compounds with up to four new contiguous stereogenic centers in a highly stereoselective and predictable manner in a single laboratory operation. − A large number of dienes and dienophiles with a plethora of functionalities has been used to construct various types of ring structures . Bicyclo[2.2.2]octenones ( 1 ), which have a wide range of applications in the synthesis of natural products, , can be accessed easily by using the Diels−Alder reaction of 2,4-cyclohexadienones with activated alkenes. , Similarly, bicyclo[2.2.2]octadienones ( 2 ) can be conveniently generated by reaction of activated alkynes with 2,4-cyclohexadienones. − Both bicyclo[2.2.2]octadienones and bicyclo[2.2.2]octenones can undergo interesting and useful photochemical reactions, viz., di-π-methane (DPM) and oxa-di-π-methane (ODPM) rearrangements, and 1,3-acyl migration and ODPM rearrangement, respectively …”

“…Unlike bicyclo[2.2.2]octenones, the only general method for preparing bicyclo[2.2.2]octadienones is the Diels− Alder reaction of 2,4-cyclohexadienones with acetylene derivatives. − Due to the limited accessibility and high propensity toward dimerization of 2,4-cyclohexadienones, their Diels−Alder reactions with acetylenes have not been studied in detail . Not surprisingly, most of the reported studies focused on reactions of dimethyl acetylenedicarboxylate with a few 2,4-cyclohexadienones bearing bulky substituents. − Furthermore, most types of 2,4-cyclohexadienones have been generated in situ from the corresponding dimers or prepared using inefficient or lengthy protocols. − Masked o -benzoquinones (MOBs) are the most easily accessible 2,4-cyclohexadienones, and the Diels−Alder chemistry of MOBs has been extensively studied in our laboratory. 5c, However, the reactions of acetylenes with MOBs bearing synthetically relevant substituents did not offer much promise. , Consequently, we focused our attention on 2,6,6-trimethyl-2,4-cyclohexadienone ( 12 ). Despite its great synthetic potential, 10b, 12 has not been widely studied, presumably due to the difficulties involved in its preparation, low yields16a or long protocols,16b and its facile dimerization. 14c,16a, Nevertheless, cyclohexadienone 12 has been used as starting material in the total synthesis of several natural products 14 such as patchouli alcohol 10b,14c,d and carotenoids (Figure ).…”

“…Not surprisingly, most of the reported studies focused on reactions of dimethyl acetylenedicarboxylate with a few 2,4-cyclohexadienones bearing bulky substituents. − Furthermore, most types of 2,4-cyclohexadienones have been generated in situ from the corresponding dimers or prepared using inefficient or lengthy protocols. − Masked o -benzoquinones (MOBs) are the most easily accessible 2,4-cyclohexadienones, and the Diels−Alder chemistry of MOBs has been extensively studied in our laboratory. 5c, However, the reactions of acetylenes with MOBs bearing synthetically relevant substituents did not offer much promise. , Consequently, we focused our attention on 2,6,6-trimethyl-2,4-cyclohexadienone ( 12 ). Despite its great synthetic potential, 10b, 12 has not been widely studied, presumably due to the difficulties involved in its preparation, low yields16a or long protocols,16b and its facile dimerization. 14c,16a, Nevertheless, cyclohexadienone 12 has been used as starting material in the total synthesis of several natural products 14 such as patchouli alcohol 10b,14c,d and carotenoids (Figure ). Therefore, we sought to develop nondimerizing and easily accessible 2,4-cyclohexadienones.…”