Applications of high-potential quinones. Part I. The mechanism of dehydrogenation of steroidal ketones by 2,3-dichloro-5,6-dicyanobenzoquinone

Abstract

The reaction of 2.3-dichloro-5.6-dicyanobenzoquinone with steroidal 4-en-3-ones is catalysed by acids. 1.2-Dehydrogenation predominates in the presence of weak acids, as it does in the uncatalysed reaction, whereas strong acids promote exclusive formation of 6,7-dehydro-derivatives. These results are rationalised by a mechanism involving oxidation of the alternative enolic forms of the ketone. Thus, in the absence of acid or with weak acids, the kinetically determined 2.4-dien-3-01 undergoes C-1 hydride ion abstraction with dichlorodicyanobenzoquinone; strong acid accelerates the formation of the thermodynamically more stable 3.5-dien-3-01. which readily suffers hydride abstraction from C-7 by the quinone. In the uncatalysed reaction the rate of 1.2-dehydrogenation depends upon both steroid and quinone concentration, whereas, in the presence of weak acids, the rate depends only on the concentration of ketone. Dehydrogenation of methylated and deuteriated 3-ketones reveals preferential elimination of the trans-diaxial 2pand 1 a-hydrogen atoms. Specific loss of the 1 a-hydrogen is ebserved with 5a-3-ones, but the oxidation of 4-en-3-ones is less specific and involves some attack a t the 1 P-position.

Facial Selectivity in the Diels–Alder Reactions of 2,2‐Disubstituted Cyclopent‐4‐ene‐1,3‐dione Derivatives and a Computational Examination of the Facial Selectivity of the Diels–Alder Reactions of Structurally Related Dienes and Dienophiles

“…[14] The dehydrogenation of 8 and 9 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) when catalyzed by acid [15] proceeded slowly, but cleanly, to provide enediones 6 and 7 (Scheme 1). Scheme 1.…”

An efficient and environmentally benign chemical synthesis of testolactone

“…For the introduction of the 1,4-diene system into the A-ring of 7, we decided to use the one-pot operation employing 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). 7 DDQ has been extensively employed in steroid chemistry avoiding the use of halogens and selenium reagents. 8 We choose to use DDQ over the palladium-catalyzed process to dehydrogenate carbonyl enol ethers 9 because we found the one-pot DDQ procedure to be more chemoselective.…”

3(5)‐(2‐Hydroxyphenyl)‐5(3)‐styrylpyrazoles: Synthesis and Diels−Alder Transformations

“…In this case it involves a (benzylic) hydride transfer from compounds 10aϪc to DDQ, and the presence of an electrondonating substituent in the phenyl ring should stabilise the formed carbocation (C-3ЈЈ). In an attempt to optimise the experimental procedure we found that the presence of a small amount (20% molar equivalent) of p-toluenesulfonic acid [16] resulted in faster formation of the expected 5-(2-hydroxyphenyl)-3-naphthylpyrazoles 12aϪc with slightly improved reaction yields (Table 1, Scheme 1, E, Method B).…”

“…Only in the oxidation of 3-(2-hydroxyphenyl)-5-{2-[3-(4-methoxyphenyl)]-1,2,3,4-tetrahydronaphthyl}pyrazole (9b) with five molar equivalents of DDQ at reflux in dry 1,4-dioxane in the presence of a small amount (20% molar equivalent) of p-toluenesulfonic acid [16] did we succeed in isolating and characterising…”