As I understand it, Kantian constructivism is partly a position in normative ethics and partly a position in metaethics. In metaethics, it is the position that ethical claims have truth values, but their truth conditions consist not in a set of objective facts to which they correspond, but instead in the outcome of some procedure of deliberation resulting in decisions about what to do.

The Russian revolution did not occur until a generation after Marx's death. He was not involved with it, or with what came after it. His works do not describe post-capitalist society, and a fortiori they do not recommend any part of what the Soviet Union did.

When people think that moral problems can be solved by some simple strategy of calculation, that sets them up for ghastly overreaching. They think they can turn everything into a "science" the way mechanics was turned into a science in the seventeeth century. They want to turn everything over to technocrats and social engineers. They become shortsighted or simplistic about their ends, and they disastrously overestimate their ability to acquire the information they need to make the needed calculations.

We can establish empirical criteria for free actions, and investigate human actions on the presupposition we are free.

I think it is clear that what we ought to do has to be independent of our decisions about what to do, and independent of any procedures we might use in making such decisions.

As Kant says, the contribution of any common laborer would be greater than that of the greatest philosopher unless the philosopher makes some contribution to establishing the rights of humanity.

Capitalism has proven to be a far more terrible system than Marx could ever bring himself to imagine. Those who are so deluded as to find something good in it, or even feel loyalty toward it, are its most pitiful victims.