Chiral polymethine dyes, IV. Synthesis, absolute configuration, spectroscopic and chiroptical properties of chiral tri‐ and pentamethinium cyanine dyes with 1,2,3,4‐tetrahydro‐6‐methylquinolyl end groups

We have repeated and significantly improved the syntheses of the symmetrical tri‐ and pentamethinium cyanine dyes 10 and 11 in their monochiral forms ( 10a, b; 11a, b ) and in the meso form ( 11d ) as well as of the unsymmetrical monochiral pentamethinium cyanine dye 14a , first realized by König and Langbein in 1928. Their spectroscopic properties have been studied for the first time. The monochiral starting compounds 4a , b , and hence all related monochiral polymethine dyes, have been obtained with high enantiomeric excess (e.e. >99%) by recrystallization of their diastereomeric 3‐bromocamphor‐8‐sulfonates 6a , b. The absolute configuration of 4a , b , and thus the absolute configuration of all monochiral polymethine dyes derived from 4a , b , has been determined by X‐ray structural analyses of the tetrahydroquinolinium perchlorate 7a and the pentamethinium bromide 11a . Partial hydrolysis of the monochiral pentamethinium dye 11a gave the monochiral pentamethine merocyanine dye 12a , which was allowed to react either with 13 to yield the unsymmetrical monochiral pentamethinium dye 14a or with monochiral 7b to afford the symmetrical achiral meso ‐pentamethinium dye 11d . The pentamethinium cyanine dyes 11a , b , d and 14a were used to examine the proposed additivity in calculating the overall molecular optical rotation of 11a , b (Table 1).