Fig. 4
From: Automatic mapping of atoms across both simple and complex chemical reactions
Application of reaction heuristics. The example in a, illustrates the consequences of applying reaction heuristic [s] (see Fig. 2) in various places of the same substrate 1 in the Prins-Pinacol rearrangement39,52. In the top row, the heuristic is properly applied to the substrate (the substructure of the substrate recognized by the heuristic is marked orange) producing intermediate 2, which is then correctly mapped into product 3 with overall score = 3.5 (3 for disconnecting bonds 5–11, 3-H, and 7-H plus 0.5 for using the heuristic). In contrast, when the same heuristic is applied to the wrong part of the molecule (1′), it yields intermediate 2′, which is incorrectly mapped into product 3′ with score = 4.5 (3 for disconnecting single bonds 4–8, 5–11, 7-H, additional 1 for converting double bond 2–3 into a single bond, and 0.5 for using the heuristic). The second example in b, illustrates the performance of the algorithm with and without the application of the metathesis heuristics (k in Fig. 3). (top row) Without the heuristics, the algorithm can identify the lowest-scoring isomorphic mapping by cutting only two single bonds—still, this solution is chemically incorrect. (bottom row) With the heuristics applied, the algorithm is not heavily (score + 4) penalized for cutting as many as four bonds (two σ and two π)—application of the heuristics costs much less (+0.5) and the algorithm can find the chemically correct isomorphic mapping
