Identification of Betulin-Related Esters in Archaeological Birch Bark Tar: Origin and Significance

Abstract

In the frame of studies dedicated to the use of pottery during the Neolithic, we have investigated an organic substance having most likely served to repair ceramics found at the site of “La Rouvière” (Rogues, Southern France) and dated to the Late Neolithic (ca. 3000 B.C.). GC-MS analysis of this substance led to its identification as birch bark tar, a material prepared by pyrolysis of birch bark and largely used as an adhesive since prehistoric times (e.g., Rageot et al., 2018; Courel et al., 2018). Along with the typical triterpenoid markers from the lupane series, four series of uncommon compounds were detected and which, based on MS interpretations, were postulated to correspond to acyl esters of lupa-2,20(29)-dien-28-ol (1, 2; numbers refer to structures in Fig. 1) and lup-2-en-28-ol (3, 4). Among these, fatty acyl esters of 1 have been previously reported by Dudd and Evershed (1999) from birch bark tar samples from Roman archaeological sites in Great Britain, but the identification remained tentative, based on MS interpretations and hydrolysis experiments. We report here the conclusive identification of the four series of compounds 1-4 using synthetic standards (1a-4a). Their origin as well as their significance with regard to the mode of preparation of the birch bark tar are discussed.