[Hockeystar]

I'm having trouble understanding the stereochemistry of the following reaction. The methyl group  between the two rings is indicated as coming out of the page and I have no idea how that happens because the ketene is linear. I know that 6e- electrocyclization happens in a disrotatory fashion, I just don't understand why the methyl group is always up.

[Irlanur]

I don't think it's actually enantioselective...

[Hockeystar]

Ok. I'll ask the prof just to clairify the answer.

[spirochete]

Optically inactive starting materials will always yield optically inactive products. At the very least, a "seed" of chirality must be placed in the form of a chiral catalyst to generate optical activity in the products. This is a fundamental concept in chemistry.

In contrast, optically active starting materials may yield either optically active or optically inactive products. Think Sn1 reaction for an example where chirality is lost, Sn2 for an example where chirality is maintained.

In the first step of the reaction the chiral center in the starting material is lost. So the chirality of the starting material, even if it is enantiomerically pure, won't be transferred to the product.

[Dan]

In the first step of the reaction the chiral center in the starting material is lost. So the chirality of the starting material, even if it is enantiomerically pure, won't be transferred to the product.

I would just like to point out that there is a chiral centre at S in the sulfoxide and (probably more importantly) the allene has axial chirality. So I'd say it's actually inevitable that stereochemical information is transferred to the product and expect it to be stereospecific.

A further comment: The starting alcohol is S-configured in the question, but the original poster has drawn a mechanism starting from the R-configured alcohol - it would appear that the centre was accidentally inverted on redrawing the starting material in a different conformation to that provided in the question.

If you work through the mechanism showing the stereochemistry of the allene, it is not too difficult to rationalise the configuration at the quaternary C centre and choose conditions for B final product (thermal or photochemical). You can also predict the stereochemistry at S in the product, which is not given in the answer (I'd go for the R-configured sulfoxide, but I will wait for the OP to have a go at it before showing my arguments).

[spirochete]

In the first step of the reaction the chiral center in the starting material is lost. So the chirality of the starting material, even if it is enantiomerically pure, won't be transferred to the product.

I would just like to point out that there is a chiral centre at S in the sulfoxide and (probably more importantly) the allene has axial chirality. So I'd say it's actually inevitable that stereochemical information is transferred to the product and expect it to be stereospecific.

Wow you're right! Can't believe I missed that.

[Dan]

Hmm, on further fiddling I think there is a mistake somewhere.

From the OP's chemdraw, I can only get to final product shown by a photochemical electrocyclisation. If the electrocyclisation is thermal (ie it is a spontaneous reaction [which I reckon it probably is] rather than a "choose your conditions" reaction), then I can only get to the final product via the enantiomeric R-alcohol (which is the configuration in the OP's hand-drawn structures).

So, either it's the chemdraw starting material is correct, the OP's first hand drawn structure incorrect, and the electrocyclisation is photochemical...

..or the chemdraw starting material is incorrect, the OP's first hand drawn structure correct, and the electrocyclisation is thermal...

...or I'm making a mistake.

Hockeystar - can you clarify this?

[orgopete]

I agree that I would not have drawn that product. I also agree that if the alcohol is chiral (and I don't know that it actually is, though it is written as though it is), the allene should be chiral. I'm expecting the sulfur to be on the same side in the allene as the starting alcohol.

My question is, "Really, only one of the two pairs of non-bonded electrons of the sulfur will react to give a chiral sulfoxide?" Or am I misinterpreting this part of the suggested mechanism?

[Dan]

My question is, "Really, only one of the two pairs of non-bonded electrons of the sulfur will react to give a chiral sulfoxide?" Or am I misinterpreting this part of the suggested mechanism?

That I don't know, but since the two transition states are diastereomeric, and therefore different in energy, I think it's reasonable to expect some transfer of stereochemical information to the sulfoxide. But I'm just guessing. The difference in energy may not be very much, I would certainly not expect 100% transfer. The fact that the configuration at S is undefined in the question may be a sign that there is little (or no) selectivity with respect to that centre.

The axial chirality of allene (which should be 100% determined by the stereochemistry of the alcohol) is what will set the absolute stereochemistry of the stereogenic quat centre formed in the electrocylcisation. I think the the sulfoxide is too remote for its configuration to affect this step - the stereochemistry of the sulfoxide is just a side point.

[orgopete]

Re: sulfur chirality

I agree that I would not expect much selectivity there and it's chirality is incidental to the chirality of the allene. I just wasn't sure if I was understanding the point being made regarding its chirality.