253. Die Chiralitãt der α‐Cinensäure

Thermal Isomerization of (+)‐cis‐ and (−)‐trans‐Pinane Leading to (−)‐β‐Citronellene and (+)‐Isocitronellene

“…As found for 4 b and 4 d and indicated in Table 1, when both groups are placed on the same side of the ring (cis configuration) the proton spectrum shows two singlets, each singlet representing one proton. [22][23][24][25][26] Otherwise (trans configuration) the 1 H NMR spectrum shows only one signal representing both =C Obviously, 4 a and 4 d differ extremely with regard to their stereochemistry, leading to the conclusion that 4 a has to have a trans,trans-configuration, whereas 4 d has to be cis,cis-configured (Table 1). [13,22] Determination in the case of 4 b an 4 c was possible upon consideration that only a total of four combinations of cis or trans are possible.…”

“…Absolute configurations were assigned by comparing the enantiomeric excess (ee) of the products of 4 with the ee of 2 used herein, assuming that their formations are enantiospecific. [13,22,26] Isomerization of 3 leads to the formation of three different products: 5 a, 5 b, and 5 c with residence times of 6.4, 7.4, and 8.0 min, respectively (Figure 1), which were separated by using preparative GC. Because of their low concentrations it was not possible to identify the structures for 5 a and 5 c, whereas the configuration of 5 b was elucidated based on (Table 1) the proton NMR spectrum showed two singlets for the =CH 2 group in 5, which enabled the conclusion that the isopropenyl group is located cis to either the methyl group at C 1 or C 2 .…”

“…According to other 1,6-dienes (2, myrcene, linalool) the formation of 5 also proceeds by ene-type cyclization (Scheme 7). [4,5,9,13,15,34,35] The presumptive transition state leading to the formation of 5 b reveals no repulsive diaxial interactions of methyl groups, explaining the high selectivity. …”

“…4, 5 in Scheme 1). [4,5,9,[13][14][15] The products from the reaction network for the thermal rearrangement based on 1 are shown in Scheme 1. According to their general structure, they can be classified into three groups assigned by three reaction pathways.…”

Thermal Behaviour of Selected C10H16 Monoterpenes

“…nopinone, pinane, pinan-2-ol) undergo cyclisation reactions involving open-chain intermediates (analogues of 2) and lead to related substances as shown in Scheme 5. [10,[26][27][28][29] From a combination of mass spectra and hydrogenation experiments it was concluded that the thermal conversion product 6 was formed from primary pyrolysis product 2 by an ene reaction. The formation of compound 5 (5-EMCHp), which was identified in the liquid pyrolysis-product mixture of substrate 1, can be explained in different ways.…”