Cinnamoyl esters of lesquerella and castor oil: Novel sunscreen active ingredients

Lesquerella and castor oils were esterified with cinnamic acid (CA) and 4‐methoxycinnamic acid (MCA). Esterification of the hydroxy oils reached 85% completion with CA and 50% conversion with MCA. The hydroxy oils were esterified at 200°C under a nitrogen atmosphere within a sealed system. Unreacted CA and MCA were removed from the reaction mixtures by sublimation at 100°C under vacuum. The resultant methoxycinnamic oils possessed a broader, more blue‐shifted UV absorbance, 250 to 345 nm with a λmax of 305 nm, compared with the cinnamic oils, which absorbed from 260 to 315 nm, λmax of 270 nm. The methoxycinnamic oils provide better UV‐B absorption and thus are better candidates to be used as sunscreen active ingredients. Esterifications of the hydroxy oils with MCA at 200°C resulted in conversion of 40% of the MCA to undesirable by‐products. Esterifications with MCA performed at 175°C in the presence of a tin catalyst resulted in similar percent conversions to product without degradation of MCA. Esterifications of lesquerella oil with MCA at 175°C resulted in higher conversions, 43%, than analogous esterifications with castor oil, 29%. The hydroxyl groups of the lesquerella and castor oils provide their excellent emolliency, lubricity, and noncomedogenicity in skin and personal‐care products. Therefore, reactions that convert only 50% of the available hydroxyl groups of the lesquerella oil to cinnamoyl‐esters are preferred.