claims that Karl Marx was anti-semitic,

That idea derives from his «Zur Judenfrage» in which he attacks Bruno Bauer who in turn attacks the Jews. Marx concedes all the terrible vices that Bruno ascribed to the Jews but proceeds to explain how the rest of the world is not any better; that the whole world had by that time adopted Bruno Bauer's brand of "Judaism". It were not the Jews doing usury, but the entire bourgeoisie. And converting the Jews to some modern day bourgeois version of christendom wouldn't magically shut down all the banks. It can well be said to be an antisemitic work because Marx enters polemic with an antisemite agreeing to his premises and to his slogans only redefining his Jews.

source:

the 1843 works of Bruno Bauer (I've only found them in German, sorry):

«The Jewish Question» book

«The Capacity of Present-day Jews and Christians to Become Free» article

the 1844 answer of Marx:

On the Jewish Question

supported slavery

I don't know which text that's ought to be. Sorry.

called for the extermination of Slavic peoples, calling them "retrograde races".

It was the Neue Rheinische Zeitung of 1848 and 1849. In it he and Engels not only slammed the Austrian Slavs, but also the Danes and Russians in the most vicious terms. It was their revolutionary rabble rousing tabloid very different in tone from all of their other works.

The Panslavists were opposed to the liberal rebels, both German and Hungarian, and the Russian tsar has helped to put them down. At that point Engels did evoke the image of a race war with the slavs.

source:

all Neue Rheinische Zeitung articles of 1848

all Neue Rheinische Zeitung articles of 1849

again: both in German.

The good folks at marxists.org have an unreasonably large collection of Marx and Engels. So it's odd to me that searching this phrase, "retrograde races," pops up only detractors with third or fourth-hand citations to old newspapers, but not to original Marxist works. And most of it is wacky New World Order or anticommunist Christian work. I'll keep looking, but I'm very skeptical he said anything of the sort.

Here is a similar phrase. The sarcasm is so clear, however, in the whole context.

In expectation, therefore, of this famous social liquidation, the working class must behave itself in a respectable manner, like a flock of well-fed sheep; it must leave the government in peace, fear the police, respect the law and offer itself up uncomplaining as cannon-fodder.

Here's Marx responding to the same accusation, interestingly enough. In this case his own bourgeoisie background is coming through a bit, but he's being hyperbolic about an internal Party political struggle.

Klein really does wield some influence over the Solingen workers, and they are the best in the Rhine province. I, pour ma part, have never, either drunk or sober, expressed the view that the workers are fit only for cannon-fodder, although the louts, among whom little Klein is evidently coming to rank himself, are, to my mind, barely fit even for that. It would be as well to treat little Klein with your accustomed discretion as a tool that may perhaps (?), in time of action, be of use to us.

I'm not finding "retrograde races" or comparable anywhere in Marx's work. Except in a fit of extreme sarcasm (which he was prone to) this doesn't match up with the rest of his philosophy. To Marx, members and cultures of the proletariat were distinguished by their time and place (following Hegel's similar line of thought) but were far more united than divided. I mean, hell, even Lenin was on that same boat:

The difference in the attitude of the two bourgeoisies was not due, of course, to the “characteristics” of different “races”, but to the different levels of economic and political development which caused one of them to fear the “younger brother”, and made it vacillate impotently between deprecating the violence of feudalism and censuring the “intolerance” of the workers.

It would be a very interesting turn of events for Marx to support the destruction of the Slavs, when panSlavism is inevitable in the discussion of Stalinism and Bolshevism at large.

Here is a letterfrom Marx congratulating Lincoln on the emancipation proclamation. I'm by no means an expert but I'm currently reading capital volume one, and in it he says that slavery, along with feudalism, is an inferior economic system because (among other reasons) they retard economic growth by disincentivizing innovation of and investment in labor saving technology. Capitalism is superior bc it is always trying to reduce labor time and thus becomes increasingly productive.