Just a couple notes from my side:
I don't believe you can lend from your IRA to yourself to flip a house since you are a "disqualified" person, or whatever the term is. Not sure what @Marty Sprong had in mind, maybe lending on someone else’s flips? I do that with my IRA, definitely a good option.
Technically you can probably do a handful of flips with an IRA, but there's some fairly steep UBIT and the potential that if you do too much it's classified as "active income" which is a no-no for an IRA. Probably best to steer clear.
A syndication would likely trigger UDFI (unrelated debt-financed income) at some point in the hold period, unless they are purchasing the asset without a loan (highly unlikely). Usually the first few years you'll get a paper loss, so no taxes to pay, but once you show a gain you'll have to pay. From what I've heard / seen, it doesn't come out to a lot, but it's a bit confusing to try and figure out yourself. I have a white paper about it from my IRA custodian that lays it out pretty clearly, I could send it your way if you shoot me a message with your email.