Structural analysis and reactivity of unusual tetrahedral intermediates enabled by SmI 2 -mediated reduction of barbituric acids: vinylogous N -acyliminium additions to a -hydroxy- N -acyl-carbamides †

Structural characterisation and reactivity of new tetrahedral intermediates based on a highly modular barbituric acid scaﬀold, formed via chemoselective electron transfer using the SmI 2 –H 2 O reagent, are reported. Lewis acid promoted cleavage of bicyclic a -amino alcohols affords vinylogous N -acyliminium ions, which undergo selective (>95:5, 1,4 over 1,2) capture with a suite of diverse nucleophiles in a practical sequence to biologically active uracil derivatives. Tetrahedral intermediates formed in the nucleophilic addition to carboxylic acid derivatives are among the most important intermediates in organic synthesis; however, only a few examples have been isolated to date due to their transient nature. 1 These species have important biological implications as models for in vivo acyl-transfer reactions. Moreover, the isolation of tetrahedral intermediates is critical for structural insight into the trajectory of nucleophilic attack onto the carbonyl group and enables the development of novel synthetic equivalents for chemoselective addition reactions with carboxylic acid derivatives. 2

Tetrahedral intermediates formed in the nucleophilic addition to carboxylic acid derivatives are among the most important intermediates in organic synthesis; however, only a few examples have been isolated to date due to their transient nature. 1 These species have important biological implications as models for in vivo acyltransfer reactions.Moreover, the isolation of tetrahedral intermediates is critical for structural insight into the trajectory of nucleophilic attack onto the carbonyl group and enables the development of novel synthetic equivalents for chemoselective addition reactions with carboxylic acid derivatives. 2icyclic uracils as direct homologues of primary nucleobases are abundant motifs in medicinal chemistry. 3Recently, derivatives of this heterocyclic scaffold have been shown to be effective as selective modulators of ionotropic glutamate receptors and exhibit activity against hepatitis C virus and HIV-1 integrase. 4While several procedures have been developed for the preparation of bicyclic uracils, 5 there is a need for robust methods for the synthesis of structurally diversified analogues (Fig. 1 and 2).
Recently, we reported the preparation of unusual tetrahedral intermediates exploiting the robust nature of the barbituric acid template. 6The success of our approach relied on a polarity reversal process utilizing SmI 2 -H 2 O as a mild electron transfer reagent operating under conditions that are fully orthogonal to other single and two-electron reaction pathways. 7,8In our initial investigation, we found that barbituric acid derived hemiaminals are thermodynamically or kinetically stable, and characterized by the beginning of collapse of the tetrahedral intermediate either by elimination of the hydroxyl group to give acyliminiums or by opening of the C-N(CQO) group to furnish alicyclic ureids.However, these initially structurally-characterised examples were limited to specific cases and did not allow for translation  of structural information into reactivity that would allow the hemiaminals to be used to access valuable uracil analogues.
Herein, we report a study on the structural characterisation of novel tetrahedral intermediates derived from the barbituric acid scaffold that demonstrates the thermodynamic stability of the a-amino alcohol function in both six-membered and heterobicyclic scaffolds.We apply these structural findings to develop a highly divergent process for the synthesis of 6-substituted-uracils via nucleophilic capture of N-acyliminium ions 9 that proceeds with exclusive (>95 : 5, 1,4 over 1,2) selectivity and demonstrate that the solid state characteristics of these hemiaminals are reflected by their solution properties.Overall, these studies set the stage for the synthesis of a wide range of diverse nitrogen-containing heterocycles and tetrahedral intermediates from cyclic scaffolds (Fig. 3). 10 The barbiturate template serves as a robust platform for the stabilization of otherwise short-lived, high-energy tetrahedral intermediates due to a combination of the stabilizing N lpp* CQO conjugation and the scaffolding effect of the six-membered ring. 6hereas steric effects were previously used to push the N lp and O lp out of conjugation with the corresponding s* C-O and s* C-N bonds, we considered that a geminal substitution of the adjacent C2-carbon atom would affect the delocalization of N lp and O lp in the a-amino alcohol moiety and provide insight into the extent of stabilization by the scaffold.After extensive investigation, we found that a very sterically-hindered spiro-cyclic hemiaminal 1, which was readily obtained using our recently reported reduction of barbituric acids by SmI 2 -H 2 O, 6 can be examined by X-ray crystallography (recryst.from acetone/water, 50 : 1 v/v, mp = 136-138 1C).
Structural parameters of 1 relevant to the geometry at the anomeric centre along with bond lengths and angles of related hemiaminals are presented in Table 1.The X-ray crystal structure of 1 reveals that the C 1 -O 1 bond (1.407 Å) is shorter than the average C sp3 -O bond (1.432 Å), while the length of the N 1 -C 1 bond is 1.463 Å, which corresponds to a typical C sp3 -N bond (1.469 Å).The C 1 -C 2 bond length of 1.522 Å is slightly shorter than the average C sp3 -C sp3 bond (1.530 Å).The torsion angle between N lp and C 1 -O 1 of 192.21 indicates a reasonably good N lps* C-O interaction.In addition, there is a good overlap between the O 1lp1 and the N 1 -C 1 bond (B1641) and between the O 1lp2 and the C 1 -H bond (B1971).These structural features indicate that the stability of the a-amino alcohol function in geminally substituted hemiaminals is thermodynamic in origin (i.e. it involves equilibrium between cyclic and alicyclic ureides as well as between hemiaminal and the corresponding iminium), thus suggesting that these systems could be applied towards generation of synthetically useful iminium ions. 9o gain insight into the extent of electronic stabilization on the N lp and O lp donation, we considered a barbituric acid hemiaminal 2 featuring an unusual allylic a-amino alcohol moiety.The X-ray structure of 2 (recryst.from acetone/water, 50 : 1 v/v, mp = 156-158 1C) is consistent with the thermodynamic stability.The torsion angle between N lp and C 1 -O 1 of 62.71 is in agreement with the absence of N lps* C-O interactions.However, there is a good overlap between the O 1lp1 and the N 1 -C 1 bond (B1651) and between the O 1lp2 and the C 1 -C 3 bonds (B1711).The N1-C1-C3-C4 torsion angle of B201 and O1-C1-C3-C4 of B1001 reveal co-planarity of the exo-cyclic double bond with the hemiaminal nitrogen.These structural features indicate electronic stabilization at the anomeric carbon and suggest the propensity of these hemiaminals to form extended N-acyliminium ions via hydroxyl elimination.The structural differences between 1 and Having identified structural features contributing to the stability of these tetrahedral intermediates, we next investigated their synthetic utility.In particular, we recognized that intermediates analogous to 2 could function as vinylogous N-acyliminium equivalents whose electrophilic character is revealed under suitably acidic conditions.We were delighted to find that upon exposure of 6a to mildly Lewis acidic conditions in the presence of allyltrimethylsilane highly regioselective formation of N-Ac-urea-enamide 7a occurred in excellent yield (Table 2, entry 1).Under these reaction conditions the 1,2-addition product was not observed. 11A broad range of nucleophiles is suitable for this 1,4-addition.The addition of small nucleophiles such as nitrile and azide occurred with excellent regiocontrol (entries 3 and 4).In the latter case loss of N 2 occurred, furnishing an a,b-unsaturated aldehyde bearing a vinylogous enamide.Propargyl and allenylsilane nucleophiles gave products with p systems ready for elaboration (entries 5 and 6).An electron deficient allylsilane afforded the product containing a halide functional handle (entry 7).Most remarkably, the addition of a very sterically-hindered ketene acetal Me 2 CQC(OMe)(OSiMe 3 ) delivered a methyl ester bearing an adjacent quaternary centre  with full control of regioselectivity (entry 8).Overall, these studies demonstrate that barbituric acid derived tetrahedral intermediates afford diverse bicyclic uracils that would be very difficult to prepare using other currently available methods.Spectroscopic studies lend additional support to the reactive properties of the a-amino alcohol moiety (Table 3 and ESI †).The solution of 1 in acetone shows a 13 C NMR peak at 84.6 ppm, which moves downfield by ca. 10 ppm in bicyclic hemiaminals such as 2, consistent with deshielding of the anomeric carbon and decomposition via C-N cleavage as the predominant pathway.IR n(CQO) frequencies indicate that the N-Ac carbonyl absorbs in the region corresponding to saturated uracil derivatives (ca.1710 cm À1 ), while the carbonyl group attached to the a-amino alcohol exhibits the reactive properties of an urea (bicyclic aminals, ca.1645 cm À1 ) or an imide (monocyclic six-membered aminals, ca.1658 cm À1 ).Thus, the spectroscopic properties correlate well with the structural stability, which decreases in the order six-membered > bicyclic hemiaminals.
In summary, tetrahedral intermediates derived from barbituric acids via electron transfer using SmI 2 -H 2 O indicate the tendency to eliminate hydroxyl and/or N-(CQO) groups to furnish N-acyliminium ions or alicyclic ureids, which is consistent with the thermodynamic stability of a-amino alcohols embedded in six-membered uracil frameworks.Studies on the synthesis of a wide range of N-containing heterocycles and tetrahedral intermediates are ongoing and will be reported shortly. 12e thank the EPRSC for financial support and James Raftery for X-ray crystallography.

Fig. 1
Fig. 1 Previous and current study: synthesis, structural characterisation and reactivity of unusual tetrahedral intermediates enabled by Sm(II).

Table 1
Selected bond lengths [Å] and angles [deg] of hemiaminals 1-5 This journal is © The Royal Society of Chemistry 2014