Lignin Model Compounds: Synthesis, Stereochemistry and Acidolysis

Sammanfattning: Lignin model compounds representative of structural elements in lignin of the b-1, b-5, b-O-4, cinnamaldehyde and cinnamyl alcohol types were synthesized. erythro-Forms of b-1 models of the 1,2-diaryl-1,3-propanediol type were prepared by BF3-catalyzed rearrangement of chalcone epoxides and subsequent reduction of the rearrangement products with NaBH4.
A threo form, threo-1,2-bis(3,4-dimethoxyphenyl)-1,3-propanediol, was synthesized by hydroboration/oxidation of (E)-2,3-bis(3,4-dimethoxy-phenyl)propenoic acid. The erythro and threo forms of the b-5 model 1-(3,4-dimethoxy-phenyl)-2-(2-hydroxy-3-methoxyphenyl)-1,3-propanediol were synthesized by rearrangement of a chalcone epoxide and subsequent reduction. Acid-catalyzed cyclization of the obtained 1,3-diol gave a b-5 model of the phenylcoumaran type, trans-2-(3,4-dimethoxyphenyl)-3-hydroxymethyl-7-methoxy-2,3-dihydrobenzo[b]f uran (small amounts of the cis isomer were present in the product). Diastereomers of models of the 1,3-propanediol type have been sterically assigned by ion exchange chromatography on an anion exchanger using a borate solution as eluent (the elution order of the diastereomers is governed by the stability of their borate complexes) in combination with 1HNMR spectral examinations of the boric acid complexes. Unambiguous steric assignments of diastereomers of several types of lignin models were accomplished by X-ray crystallography, stereoselective synthesis and NMR spectral comparisons. The same methods were applied to the assignment of diastereomers of conversion products of lignin models (stilbenes, 2,3-diaryl-2-propen-1-ols, quinone methides). On acidolysis, the b-1 model 1,2-bis(3,4-dimethoxyphenyl)-1,3-propanediol underwent two competing reactions: a reverse Prins reaction leading to trans-3,3¢,4,4¢-tetramethoxystilbene (and formaldehyde) and a dehydration with formation of aryl-substituted allyl alcohols which, subsequently, were converted into a series of carbonyl compounds and 2-(3,4-dimethoxyphenyl)-5,6-dimethoxy-1H-indene (phenolic b-1 models were shown to react analogously). HBr (and to a lesser extent HCl) favored the formation of the allyl alcohols while only traces of these compounds were formed when CH3SO3H was used as the catalyst. Acidolysis of the b-5 model trans-2-(3,4-dimethoxyphenyl)-3-hydroxymethyl-7-methoxy-2,3-dihydrobenzo[b]f uran gave primarily a 2-arylbenzofuran when HBr was used as catalyst while a trans-stilbene derivative was the predominant product when CF3SO3H was used as catalyst; both acids catalyzed an isomerization of the starting material leading to the formation of small amounts of the cis isomer.