Stereocomplementary Syntheses of 1,ω‐DistannylatedE,Z‐Isomeric Conjugated Trienes, Tetraenes, and Pentaenes

Abstract

Stereoselective syntheses of 1,6-bis(tributylstannyl)hexa-1,3,5-trienes, 1,8-bis(tributylstannyl)octa-1,3,5,7-tetraenes, and 1,10-bis(tributylstannyl)deca-1,3,5,7,9-pentaenes with various methylation patterns were achieved based on stereocomplementary C=C bond-forming reactions. All-E isomers resulted from Ramberg-Bäcklund rearrangements of distannylated diallyl-, allylpentadienyl-, or bis- (pentadienyl)sulfones. Mono-Z-configured 1,ω-bis(tributylstannyl)-1,3,5-polyenes emerged from (Sylvestre-)Julia olefinations of Bu(3)Sn-substituted enals or dienals with Bu(3)Sn-substituted allyl or pentadienyl benzothiazolylsulfones. Related Ramberg-Bäcklund approaches provided all-E-1-bromo-6-(tributylstannyl)hexa-1,3,5-triene but not all-E-1-(tetramethyldioxaborolanyl)-6-(tributylstannyl)hexa-1,3,5-triene.

Synthesis of Symmetrical and Nonsymmetrical Polyenes by Iterative and Bidirectional Palladium‐Catalyzed Cross‐Coupling Reactions

“…Although all‐ trans ‐hexa‐1,3,5‐triene functionalized at both termini as pinacolboranes, namely 45 (Figure 2) has been reported, [6b] its potential synthetic utility could not be further explored due to the lack of reactivity observed upon attempted Suzuki–Miyaura cross‐coupling with 5‐(bromomethylene)‐(5 H )‐furan‐2‐one (see 80 , Scheme 8). [6b] …”

“…The isomeric ( E , E , E )‐trienylbisstannanes (Figure 2, Scheme 7) were instead made by Ramberg–Bäcklund rearrangement of the bisstannylated sulfones in THF, which took place in general with good E > Z selectivity (from 80:20 to 100:0, depending upon the substitution pattern, in 43 a – e , Figure 2). [6b] For example, (1 E ,3 Z ,5 E )‐1,6‐bis(tributylstannyl)hexa‐1,3,5‐trienes 44 b and 44 d were made with high stereoselectivity (95:5 d.r.) in 56 and 64 % yields, respectively, from (methyl‐substituted) stannylated α,β‐unsaturated aldehydes 73 / 74 and methyl‐substituted stannylated allyl BT sulfone 72 using the Julia–Kocienski reaction with KHMDS as base under Barbier conditions [48] .…”

“…in 56 and 64 % yields, respectively, from (methyl‐substituted) stannylated α,β‐unsaturated aldehydes 73 / 74 and methyl‐substituted stannylated allyl BT sulfone 72 using the Julia–Kocienski reaction with KHMDS as base under Barbier conditions [48] . Precursor allyl‐BT sulfone 72 was synthesized using the Mukaiyama redox condensation reaction of allylbromide 70 with diisopropyl azodicarboxylate (DIAD), triphenylphosphine and benzo‐1,3‐thiazol‐2‐thiol 71 , followed by oxidation of the sulfide with H 2 O 2 and catalytic quantities of the peroxomolybdate(VI) reagent Mo 7 (NH 4 ) 6 O 24 in EtOH [6b] …”

“…Non‐symmetrical Ramberg–Bäcklund bis‐allylsulfone 78 was prepared in an overall 62 % yield by hydrolysis of precursor thioacetate 75 with KOH and treatment of the resulting thiolate with allyl bromide 76 followed by the Mo VI ‐catalyzed oxidation of the sulfide with H 2 O 2 . Alternatively, the symmetrical sulfone 79 was prepared by reaction of allyl bromide 77 with Na 2 S⋅9H 2 O promoted by Bu 4 N + HSO 4 − , and oxidation as above in 62 % overall yield [6b] . The subsequent rearrangement of bis‐allylsulfones 78 and 79 to trienylstannanes 43 b and 43 d , respectively, took place in short reaction times upon treatment of CH 2 Cl 2 solutions with CBr 2 F 2 and KOH in Al 2 O 3 .…”

“…Yields were higher than 80 %, and the relative ratio of these geometric isomers was greater than 94:6 (Scheme 7). [6b] …”