How to label atoms or groups “backwards” in ChemDraw 16.0 ?ex: while trying to draw L-tyrosine with the hydroxyl part of the phenol as HO instead of OH as it appears in the ChemDraw image.

How to label atoms or groups “backwards” in ChemDraw 16.0 ?
ex: while trying to draw L-tyrosine with the hydroxyl part of the phenol as HO instead of OH as it appears in the ChemDraw image.

When labelling an atom in text mode, ChemDraw will automagically assume a label from left to right (i.e. in standard North American/European writing direction) and interpret the label as such.

If it finds that the atom has bonds to the right but none to the left, as tyrosine’s hydroxy group has in your example, it will group the label into ‘sensible’ fragments[1] and reverse them.
This makes sense if you think of rotating a molecule: If you had drawn tyrosine rotated by 180°180°, you would have wanted to add OH on the rightmost atom, not HO. If you then manually rotate the entire molecule within ChemDraw, you will notice the OH flipping into an HO as soon as it graphically makes sense to do so.

Note that if you just hover over the atom in lasso mode or bond adding mode and then hit o(lowercase!) it will automatically add OH (or HO, depending on the orientation) at that position. Hitting o again will remove the additional hydrogen (this holds true for all chemical elements that are attached to single letters regardless whether the atom is attached to a lowercase letter (i.e. o) or an uppercase one (B for boron)).

Consider OTBS: ChemDraw automatically recognises TBS as an abbreviation for the TBSTBSgroup so when reversing, it becomes TBSO. However ChemDraw does not standardly recognise the triethylsilyl group TESTES, so when reversing OTES it arrives at SETO. The preferred way would be to add an abbreviation to ChemDraw.

Note that you can view the known nicknames using File>List Nicknames. You can define your own nicknames using Structure>Define Nickname. Also note that ChemDraw breaks atom labels (not nicknames) into tokens that consist of an uppercase letter followed by any numbers or lowercase letters. Therefore, -OH-OH is flipped to HO-HO-, whereas -Br-Br is not flipped to rB.