Chemdraw (CDX) import fails to yield complete molecules for implicit alkyl chains

Question

Mathematica has supported Chemdraw file (CDX) import since 12.1. However, there appear to be an error when side chains are defined in an abbreviated fashion (as is common in organic chemistry):

Consider the following example Chemdraw file:

url = "https://www.oecd-nea.org/ideal/structures/BuBTBP.cdx?fileKey=238";
Import[url]
%["MolecularFormulaString"]
MoleculePlot[%%]

(*"C16H6*4N8"*)

Notice how the resulting molecule has some of the positions replaced with * characters.

If I instead download the file at that URL and open it in the Chemdraw application, I see the following:

(Note how the side chains are defined as implicit butyl groups

Selecting Copy As...>SMILES in ChemDraw puts the following string into the Clipboard, which I can paste in Mathematica to get the correct final molecule, molecular formula, etc:

chemdrawCopyAs = "CCCCC(N=N1)=C(CCCC)N=C1C2=NC(C3=NC(C4=NN=C(CCCC)C(CCCC)=N4)=CC=C3)=CC=C2";
Molecule[%]
%["MolecularFormulaString"]
MoleculePlot[%%]

(*"C32H42N8"*)

Answer 1

ChemDraw can interpret the correct representation, even if this is not explicitly represented in the CDX file. So the idea is to remotely control ChemDraw to get the information, and to do that we have to build three pieces of functionality:

1. Drive ChemDraw with AppleScript to open a CDX file, select all of the molecules (there should only be one...), and Edit>Copy As...>SMILES

2. Retrieve the results from the clipboard and use this to populate the entry in our record (with a sanity check against the stated molecular weight and molar mass)

3. Automate downloading the CDX files from the URLs, putting the results into a temporary directory to process them

This type of system-level application scripting is going to be very operating system dependent, so the solution below will only work on a Mac; for the record I performed this with MacOS 13.5 and ChemDraw 22.2 (and Mathematica 13.3).

We start by creating a lightweight function wrapper around the osascript command line program which will let us run AppleScripts; this codes is copied directly from a Stack Exchange post on using AppleScript in Mathematica:

(* Code source: https://mathematica.stackexchange.com/questions/36764/how-to-import-a-numbers-spreadsheet/36772#36772 *)
AppleScript["RunFile", file_] := Run["osascript " <> file]
AppleScript["RunScript", script_] := With[
     {file = ToFileName[$TemporaryDirectory, "script.txt"]}, 
     Export[file, script, "String"]; 
     AppleScript["RunFile", file] 
    ]

After a bit of fumbling around, I devised the following AppleScript which takes a file path (provided as the typical POSIX filepath string) as input and results in the SMILES string on the clipboard. Once you know how to specify keystroke commands in AppleScript, it is relatively straightforward. After getting the script to run in the ScriptEditor, I created the following lightweight function wrapper using a StringTemplate:

scriptTemplate[file_] := AbsoluteFileName[file] // StringTemplate["set p to \"``\"set f to POSIX file ptell application \"ChemDraw\"    open f  activate    tell application \"System Events\"      keystroke \"a\" using {command down}        keystroke \"c\" using {option down, command down}       keystroke \"w\" using {command down}    end tell    delay 0.5end tell"]

(I found that it worked more reliably if I inserted a short delay after the copy and paste operation.) We also need to retrieve the result from the clipboard, as described in this stackoverflow thread (including versions for Windows and Ubuntu; the version implemented below is for MacOS):

(* Code source: https://mathematica.stackexchange.com/a/130224/63709 *)
  Clear[getSMILESFromCDX] 
   getClipboard[___] := Import["!pbpaste", "Text"]
getSMILESFromCDX[file_] := getClipboard @ AppleScript["RunScript", scriptTemplate[file]]
(demo) 
   getSMILESFromCDX["~/Downloads/BuBTBP.cdx"]
("CCCCC(N=N1)=C(CCCC)N=C1C2=NC(C3=NC(C4=NN=C(CCCC)C(CCCC)=N4)=CC=C3)=CC=C2")

Finally, we want to be able to retrieve a file from an URL, analogous to Import. We will write a custom function using URLDownload returns a File to a temporary download directory):

importMolFromURL[url_] := Molecule @ getSMILESFromCDX @ URLDownload[url]
(demo) 
url = "https://www.oecd-nea.org/ideal/structures/BuBTBP.cdx?fileKey=238"; 
importMolFromURL[url]