Simplified Preparation of Coniferyl and Sinapyl Alcohols HOON KIM* ,† AND JOHN RALPH § U.S. Dairy Forage Research Center, Agricultural Research Service, U.S. Department of Agriculture, Madison, Wisconsin 53706, and Department of Forestry, University of WisconsinsMadison, Madison, Wisconsin 53706 Coniferyl and sinapyl alcohols were prepared from commercially available coniferaldehyde and sinapaldehyde using borohydride exchange resin in methanol. This reduction is highly regioselective and exceptionally simple, making these valuable monolignols readily available to researchers lacking synthetic chemistry expertise. KEYWORDS: Coniferyl alcohol; sinapyl alcohol; coniferaldehyde; sinapaldehyde; lignin; borohydride exchange resin; Amberlite IRA-400 INTRODUCTION The synthesis of high-quality monolignols, the 4-hydroxy- cinnamyl alcohols (coniferyl and sinapyl alcohols), has been an important step for successful preparation of lignans, synthetic lignin model compounds, and synthetic lignins or dehydroge- nation polymers (DHPs). Many multistep synthetic methods have been developed to prepare such monolignols from various commercially available starting materials (1-6). Reductions of ferulate and sinapate with lithium aluminum hydride (1, 7, 8), sodium bis(2-methoxyethyl) aluminum hydride (Red-Al) (9, 10), or diisobutyl aluminum hydride (DIBAL-H) (11) have all been used to prepare 4-hydroxycinnamyl alcohols. Since 4-hydroxycinnamaldehydes (coniferaldehyde and sinap- aldehyde) have become commercially available, several boro- hydride reagents have been used to prepare 4-hydroxycinnamyl alcohols by one-step reductions instead of the multistep syntheses previously used. Recently, the reduction of 4-hydroxy- cinnamaldehydes with sodium borohydride (12, 13) and sodium triacetoxyborohydride (14-16) was examined (17, 18). All of the described methods require handling moderately reactive and toxic reagents, and the quality of the product depends on nontrivial workup steps. Time-consuming reactions can also be a frustration. Since Gibson and Baily first reported the preparation and use of borohydride exchange resin (BER) in 1977 (19), it has been used to reduce a variety of functional groups. Regioselective reduction of carbonyl compounds in alcoholic solvents is one of its most important uses (20, 21). BER provides many of the practical advantages of other polymer-supported reagents in organic reactions (22, 23). BER is more stable than sodium borohydride itself and easier to handle (24). Coniferyl and sinapyl alcohols are commercially available now, but are 8-10 times more expensive than 4-hydroxycin- namaldehydes. Even when the expenses for resin and solvents are considered, the total cost of synthesizing the 4-hydroxycin- namyl alcohols is 2-3 times lower than for the commercial products. More importantly, the purchased products are some- times of vastly inferior quality, often containing degradation products, particularly in the case of sinapyl alcohol. Obliviously using these compounds without carefully checking their purity first has caused problems for several researchers. Here we report a simple protocol that produces clean coniferyl or sinapyl alcohol from coniferaldehyde or sinapaldehyde using BER. This method is efficient and can be safely performed without complication within short periods of time to produce fresh, clean 4-hydroxycinnamyl alcohols. EXPERIMENTAL METHODS General. NMR analyses of the coniferyl and sinapyl alcohols in acetone-d6 were performed on a Bruker Avance-360 spectrometer. The central acetone solvent peak was used as the internal reference ( 1 H, 2.04 ppm; 13 C, 29.80 ppm). The NMR data were identical to those previously reported (11) and available in the lignin model compound database (25). Coniferaldehyde, sinapaldehyde, and “borohydride, polymer supported” [borohydride exchange resin, BER; 2.5-5.0 mmol of BH - 4/g of resin; the borohydride (BH - 4) concentration was assumed as 2.5 mmol/g of resin for calculations, even though the reagent had a designated concentration of 2.5-5.0 mmol of BH - 4/g of resin] were purchased from Aldrich (Milwaukee, WI). All solvents were purchased from Fisher Scientific (Pittsburgh, PA) unless otherwise noted. Silica gel 60 (particle size ) 0.040-0.063 mm, 230-400 mesh ASTM) was purchased from EMD Chemicals Inc. (Gibbstown, NJ). Solid-phase extraction (SPE) tubes (Supelclean, LC-SI, 3 mL) were purchased from Supelco (Bellefonte, PA). The 4-hydroxycinnamyl alcohol products were examined by GC-MS. GC (ThermoQuest Trace GC 2000) conditions were as follows: Zebron ZB-5 column (30 m × 0.25 mm, 25 µm film thickness, Phenomenex); initial column temperature, 220 °C, held for 1 min, ramped at a rate of 4 °C/min to 248 °C, then ramped at rate of 30 °C/min to 300 °C for 20 min; inlet temperature, 300 °C. MS (ThermoQuest GCQ/Polaris MS) conditions were as follows: ion * Address correspondence to this author at the U.S. Dairy Forage Research Center, 1925 Linden Dr. W., Madison, WI 53706-1108 [telephone (608) 890-0055, (608) 890-0073; fax (608) 890-0076; e-mail hoonkim@ wisc.edu]. † U.S. Dairy Forage Research Center. § Department of Forestry, University of WisconsinsMadison. J. Agric. Food Chem. 2005, 53, 3693−3695 3693 10.1021/jf047787n CCC: $30.25 © 2005 American Chemical Society Published on Web 04/09/2005