Journal of the American Chemical Society Scheme 6. Crystallization of Bis-(2-aminothiazole)-aminals of Glycolaldehyde 3 and D-Glyceraldehyde 22 from Water at pH 7 the change in solubility properties was due to the increased number of hydroxyl moieties or perhaps due to the diﬀerential solubility and packing properties of rac-27 with respect to the achiral aminal 25, we investigated the solubility properties of the aminal structures formed in a mixture of D-glyceraldehyde 22 and thiazole 7. Intriguingly, we found that at 100 mM concentration, D-27 now crystallized from water at 20 °C. NMR analysis showed a 1:2 ratio of 22/7 and recrystallization (10:1 H2O/EtOH) allowed for crystallographic conﬁrmation of the aminal structure (Scheme 6).25 ■ SUMMARY In summary, our results suggest that in a sulfur-rich prebiotic environment, oxazole 1 and thiazole 7 could be formed together under the same conditions, as long as both glycolaldehyde 3 and its thiol analogue, β-mercapto-acetalde- hyde 6, were present along with cyanamide 2. In such mixed reactions, 2-aminothiazole 7 forms ﬁrst, followed by 2- aminooxazole 1. The absence of any interference between oxazole and thiazole synthesis shows that these potential precursors of deoxynucleotides and ribonucleotides could indeed be formed at the same time in the same conditions. We have also found that two abiotically plausible methods for the puriﬁcation of oxazole 1, namely, crystallization and sublimation, also apply to thiazole 7. However, the chemical reactivities of oxazole 1 and thiazole 7 are quite diﬀerent, suggesting that the subsequent pathways would diverge. The most striking diﬀerence between the reactivity of oxazole 1 and thiazole 7 is the failure of the thiazole to react with aldehydes to form the thiazoline precursors of the (thio)- pyrimidine nucleotides. The most likely explanation for this diﬀerence is the increased aromaticity of the thiazole relative to the oxazole, and thus decreased nucleophilicity of C5. We are currently investigating an alternative synthesis of 2′-deoxyribo- pyrimidine nucleotides mediated by C2′-inversion of anhy- dronucleotide 28 (Scheme 1).1a,11 However, during the course of eﬀorts to observe two-component reactivity of thiazole 7 with aldehydes, we observed instead the crystallization of glycolaldehyde 3 and homochiral (but not racemic) glycer- aldehyde 22 (two known ribonucleotide precursors)1,3 as aminals directly from water. The facile reversibility of aminal formation suggests that the chemical sequestration and protection of C2- and C3-aldehyde nucleotide synthons by another important class of nucleotide synthon (azoles) may have provided a route to the concentration, puriﬁcation, and stabilization of the necessary aldehyde precursors of nucleo- tides. Although these aldehydes can in principle be readily synthesized by a variety of routes,26,27 their high reactivity has Article until now made it diﬃcult to conceive of means for accumulating large reservoirs of these essential starting materials in a prebiotic context. By exploring the multicomponent reactivity of thiazole 7, together with aldehydes and the purine precursor amino- imidazole 16, we have demonstrated a selective and high yielding 3-component carbon−carbon bond-forming reaction that chemospeciﬁcally furnishes masked-2′-thiosugars regiospe- ciﬁcally tethered to an amino-imidazole purine-precursor. The facial selectivity of thiazole 7, upon addition to α- hydroxyaldehyde-imines, is quite relaxed (relative to oxazole 1), resulting in low diastereoselectivity at C2′; however, this is inconsequential with regards to the proposed deoxynucleotide synthesis because C2′ would be rendered achiral following desulfurization. In contrast, we observed very high diaster- eoselectivity of nucleophilic addition of thiazole 7 to imine 21, generating almost exclusively the lyxo and xylo isomers with controlled N9−C1′ annulation. The conversion of the resulting C5-thiazolines to 2′-deoxyribo-purine nucleotides requires three additional steps: C3′-inversion to release the purine-precursor from its tethered attachment to C3′, completion of the purine heterocycle, and chemospeciﬁc desulfurization of C2′.28 These steps are currently under investigation in our laboratories as part of our continuing studies aimed at the abiotic synthesis of 2′-deoxyribonucleotides.29 Finally, it is of note that if life emerged in an environment containing both ribo- and 2′-deoxyribonucleotides, it is likely that the ﬁrst biopolymers were heterogeneous in composition, that is, composed of a mixture of ribo- and deoxyribonucleo- tides, and perhaps other compatible nucleotides. Recent work has shown that functional biopolymers, in the form of aptamers with highly speciﬁc molecular recognition properties, can be derived by in vitro evolution from libraries of polynucleotides composed of randomly interspersed ribo- and deoxyribo- nucleotides.29 Thus, chimeric RNA/DNA polymers may have been suﬃcient for the emergence of life. The primordial biochemical exploitation of a mixed RNA and DNA genetic system could eliminate the requirement for a genetic takeover (of RNA by DNA), and would arguably result in a simpliﬁcation of the transition from chemistry to biology. However, such mixed-composition polymers have neither the advantageous stability of DNA, nor the optimal functional characteristics of RNA. Thus, RNA and DNA may have emerged as early and contemporaneous specializations of a primitive mixed biopolymer. ■ EXPERIMENTAL SECTION General Methods. Reagents and solvents were purchased from Sigma-Aldrich, TCI America, Frontier Scientiﬁc, or Cambridge Isotope Laboratories. Flash column chromatography was carried out on Merck 9385 silica gel 60 (230−400 mesh). NMR spectroscopy was carried out on a Varian NMR spectrometer (Oxford AS-400) or a Bruker NMR spectrometer (AvanceIII 600) operating at 25 °C probe temperature (unless otherwise speciﬁed). When possible, the chemical shift of the corresponding solvent was used as a reference. Chemical shift values are reported in parts per million (ppm) and J-couplings are recorded in hertz (Hz). Electrospray mass spectrometry was recorded on a Bruker Daltonics Esquire 6000 ESI-MS. High resolution mass spectrometry was carried out on a Waters Q-ToF micro LC/MS/MS system. Infrared spectra were recorded as manually pressed KBr discs on a PerkinElmer spectrum 100 series FT-IR spectrometer. Single crystal X-ray crystallography was carried out with a Bruker APEX II CCD diﬀractometer (Mo Kα radiation, λ = 0.71073 Å), equipped with an Oxford Cryosystems nitrogen ﬂow apparatus, at 100 K. Data 13892 dx.doi.org/10.1021/ja306176n | J. Am. Chem. Soc. 2012, 134, 13889−13895