Incorporation of an Allene Unit intoα-Pinenevia β-Elimination

Abstract

The two double-bond isomers 3-iodo-2,6,6-trimethylbicyclo[3.1.1]hept-2-ene (6b) and 3-iodo-4,6,6-trimethylbicyclo[3.1.1]hept-2-ene (11) were synthesized by reacting 2,6,6-trimethylbicyclo[3.1.1]heptan-3-one (9) with hydrazine, followed by treatment with I 2 in the presence of Et 3 A C H T U N G T R E N N U N G N. Treatment of 11 with t-BuOK as base in diglyme at 2208 resulted in the formation of 9 and 6,6-dimethyl-4-methylidenebicyclo[3.1.1]hept-2-ene (12). For the formation of 9, the cyclic allene 7 is proposed as an intermediate. Treatment of the second isomer, 6b, with t-BuOK at 1708 gave rise to the diene 12 and the dimerization product 17. The underlying mechanism of this transformation is discussed. On the basis of density-functional-theory (DFT) calculations on the allene 7 and the alkyne 15, the formation of the latter as the intermediate was excluded.Introduction. -The synthesis of cyclic allenes with eight or less skeletal C-atoms, known as highly strained organic compounds, has for the past decades attracted increasing interest [1]. Besides synthetic considerations, theoretical chemists have been keen on investigating these compounds to obtain insight into their structural and unusual physico-chemical properties [2]. Strained cyclic allenes are nonplanar and chiral rather than planar zwitterionic or carbene-like species, even in the case of the highly strained cyclohexa-1,2-diene and cyclohepta-1,2-diene [3].Among the numerous synthetic approaches [1] to cyclic allenes, the DoeringMoore-Skattebol method and the b-elimination method have been most widely studied in the literature. The first one, discovered by Moore [4] and co-workers, and by Skattebol [5], involves the conversion of 1,1-dihalocyclopropanes [6] to the corresponding cyclic allenes upon treatment with alkyllithium reagents [7] [8]. The latter -first attempted by Favorskii [9] to prepare cyclopenta-1,2-diene by treatment of vinyl bromide with t-BuOK -is the reaction of the corresponding vinyl halides with bases (for recent examples, see [10]).More recently, we have reported both experimental and theoretical studies related to the Doering-Moore-Skattebol reaction to generate a cyclic allene incorporated into a-pinene (1) [8]. As shown in Scheme 1, four products were isolated, the major being the carbene-insertion product 4; the others were derived from allene dimerization.Although the Doering-Moore-Skattebol method was successful in obtaining the desired allene, it gave the ring-enlarged product 5 where the allene bonds are located in a seven-membered ring. Hence, incorporation of an allene unit into the a-pinene (1) skeleton without ring enlargement would generate the six-membered cyclic allene 7 (Scheme 2), which would cause considerable deviation from linear geometry. In this

New Bio‐Polyamides from Terpenes: α‐Pinene and (+)‐3‐Carene as Valuable Resources for Lactam Production

“…The initial oxidation of the double bond to alcohols 2a and 2b was performed by hydroboration, and the anti‐Markovnikov structures were obtained selectively in good yields of up to 82% . The purity was sufficient for the subsequent reactions, and further purification was not required.…”

“…The purity was sufficient for the subsequent reactions, and further purification was not required. Although the conversion to ketones 3a and 3b has been previously performed with chromium salts, to avoid hazardous chemicals we used an adapted Oppenauer reaction reported by Christopher R. Graves et al for the oxidation of the monoterpene hydroxyborneol . We replaced the usually applied redox equivalent acetone with 3‐nitrobenzaldehyde (3‐NBA).…”

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“…(+)-isopinocamphone ((1 R ,2 R ,5 S )-2,6,6-Trimethylbicyclo[3.1.1]heptan-3-one) was synthesized as described in [105,106]. A mixture of (1 R ,2 R ,3 R ,5 S )-(−)-isopinocampheol (200 mg, 1.30 mmol, Sigma-Aldrich) and Dess-Martin-periodinane (825 mg, 1.95 mmol) in anhydrous CH 2 Cl 2 (15 mL) was stirred at room temperature for 1 hour, followed by the addition of water and sat.…”

Recent advances in the syntheses, transformations and applications of 1,1-dihalocyclopropanes

“…30 The allene formed was trapped with furan; the methoxy derivative gave only the exo product whereas the bromo derivative gave the exo and endo products in the ratio 4 : 1 (Scheme 62). 30 The allene formed was trapped with furan; the methoxy derivative gave only the exo product whereas the bromo derivative gave the exo and endo products in the ratio 4 : 1 (Scheme 62).…”